[feature] User-selectable preset CSS themes for accounts (#2777)

* [feature] User-selectable preset themes

* docs, more theme stuff

* lint, tests

* fix css name

* correct some little issues

* add another theme

* fix poll background

* okay last theme i swear

* make retrieval of apimodel themes more conventional

* preallocate stylesheet slices

18 files changed, 496 insertions, 21 deletions

diff --git a/internal/api/client/accounts/accounts.go b/internal/api/client/accounts/accounts.go
index c94fbfbaa..8b92bd7a5 100644
--- a/internal/api/client/accounts/accounts.go
+++ b/internal/api/client/accounts/accounts.go
@@ -55,6 +55,7 @@ const (
 	VerifyPath        = BasePath + "/verify_credentials"
 	MovePath          = BasePath + "/move"
 	AliasPath         = BasePath + "/alias"
+	ThemesPath        = BasePath + "/themes"
 )
 
 type Module struct {
@@ -114,4 +115,7 @@ func (m *Module) Route(attachHandler func(method string, path string, f ...gin.H
 	// migration handlers
 	attachHandler(http.MethodPost, AliasPath, m.AccountAliasPOSTHandler)
 	attachHandler(http.MethodPost, MovePath, m.AccountMovePOSTHandler)
+
+	// account themes
+	attachHandler(http.MethodGet, ThemesPath, m.AccountThemesGETHandler)
 }
diff --git a/internal/api/client/accounts/accountupdate.go b/internal/api/client/accounts/accountupdate.go
index 02c0fc5ac..905d11479 100644
--- a/internal/api/client/accounts/accountupdate.go
+++ b/internal/api/client/accounts/accountupdate.go
@@ -309,6 +309,7 @@ func parseUpdateAccountForm(c *gin.Context) (*apimodel.UpdateCredentialsRequest,
 			form.Source.Language == nil &&
 			form.Source.StatusContentType == nil &&
 			form.FieldsAttributes == nil &&
+			form.Theme == nil &&
 			form.CustomCSS == nil &&
 			form.EnableRSS == nil) {
 		return nil, errors.New("empty form submitted")
diff --git a/internal/api/client/accounts/lists.go b/internal/api/client/accounts/lists.go
index d42fdd3f9..7bd1227a8 100644
--- a/internal/api/client/accounts/lists.go
+++ b/internal/api/client/accounts/lists.go
@@ -69,7 +69,7 @@ import (
 //		'500':
 //			description: internal server error
 func (m *Module) AccountListsGETHandler(c *gin.Context) {
-	authed, err := oauth.Authed(c, false, false, false, false)
+	authed, err := oauth.Authed(c, true, true, true, true)
 	if err != nil {
 		apiutil.ErrorHandler(c, gtserror.NewErrorUnauthorized(err, err.Error()), m.processor.InstanceGetV1)
 		return
diff --git a/internal/api/client/accounts/statuses.go b/internal/api/client/accounts/statuses.go
index 7dd4cbe37..a72a464ed 100644
--- a/internal/api/client/accounts/statuses.go
+++ b/internal/api/client/accounts/statuses.go
@@ -134,7 +134,7 @@ import (
 //		'500':
 //			description: internal server error
 func (m *Module) AccountStatusesGETHandler(c *gin.Context) {
-	authed, err := oauth.Authed(c, false, false, false, false)
+	authed, err := oauth.Authed(c, true, true, true, true)
 	if err != nil {
 		apiutil.ErrorHandler(c, gtserror.NewErrorUnauthorized(err, err.Error()), m.processor.InstanceGetV1)
 		return
diff --git a/internal/api/client/accounts/themesget.go b/internal/api/client/accounts/themesget.go
new file mode 100644
index 000000000..5a0cb6d94
--- /dev/null
+++ b/internal/api/client/accounts/themesget.go
@@ -0,0 +1,77 @@
+// GoToSocial
+// Copyright (C) GoToSocial Authors admin@gotosocial.org
+// SPDX-License-Identifier: AGPL-3.0-or-later
+//
+// This program is free software: you can redistribute it and/or modify
+// it under the terms of the GNU Affero General Public License as published by
+// the Free Software Foundation, either version 3 of the License, or
+// (at your option) any later version.
+//
+// This program is distributed in the hope that it will be useful,
+// but WITHOUT ANY WARRANTY; without even the implied warranty of
+// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+// GNU Affero General Public License for more details.
+//
+// You should have received a copy of the GNU Affero General Public License
+// along with this program.  If not, see <http://www.gnu.org/licenses/>.
+
+package accounts
+
+import (
+	"net/http"
+
+	"github.com/gin-gonic/gin"
+	apiutil "github.com/superseriousbusiness/gotosocial/internal/api/util"
+	"github.com/superseriousbusiness/gotosocial/internal/gtserror"
+	"github.com/superseriousbusiness/gotosocial/internal/oauth"
+)
+
+// AccountThemesGETHandler swagger:operation GET /api/v1/accounts/themes accountThemes
+//
+// See preset CSS themes available to accounts on this instance.
+//
+//	---
+//	tags:
+//	- accounts
+//
+//	produces:
+//	- application/json
+//
+//	security:
+//	- OAuth2 Bearer:
+//		- read:accounts
+//
+//	responses:
+//		'200':
+//			name: statuses
+//			description: Array of themes.
+//			schema:
+//				type: array
+//				items:
+//					"$ref": "#/definitions/theme"
+//		'400':
+//			description: bad request
+//		'401':
+//			description: unauthorized
+//		'404':
+//			description: not found
+//		'406':
+//			description: not acceptable
+//		'500':
+//			description: internal server error
+func (m *Module) AccountThemesGETHandler(c *gin.Context) {
+	_, err := oauth.Authed(c, true, true, true, true)
+	if err != nil {
+		apiutil.ErrorHandler(c, gtserror.NewErrorUnauthorized(err, err.Error()), m.processor.InstanceGetV1)
+		return
+	}
+
+	if _, err := apiutil.NegotiateAccept(c, apiutil.JSONAcceptHeaders...); err != nil {
+		apiutil.ErrorHandler(c, gtserror.NewErrorNotAcceptable(err, err.Error()), m.processor.InstanceGetV1)
+		return
+	}
+
+	// Retrieve available themes.
+	themes := m.processor.Account().ThemesGet()
+	apiutil.JSON(c, http.StatusOK, themes)
+}
diff --git a/internal/api/model/account.go b/internal/api/model/account.go
index 7d3fa4b62..af2a394af 100644
--- a/internal/api/model/account.go
+++ b/internal/api/model/account.go
@@ -89,6 +89,8 @@ type Account struct {
 	MuteExpiresAt string `json:"mute_expires_at,omitempty"`
 	// Extra profile information. Shown only if the requester owns the account being requested.
 	Source *Source `json:"source,omitempty"`
+	// Filename of user-selected CSS theme to include when rendering this account's profile or statuses. Eg., `blurple-light.css`.
+	Theme string `json:"theme,omitempty"`
 	// CustomCSS to include when rendering this account's profile or statuses.
 	CustomCSS string `json:"custom_css,omitempty"`
 	// Account has enabled RSS feed.
@@ -162,7 +164,11 @@ type UpdateCredentialsRequest struct {
 	FieldsAttributes *[]UpdateField `form:"fields_attributes" json:"-"`
 	// Profile metadata names and values, parsed from JSON.
 	JSONFieldsAttributes *map[string]UpdateField `form:"-" json:"fields_attributes"`
+	// Theme file name to be used when rendering this account's profile or statuses.
+	// Use empty string to unset.
+	Theme *string `form:"theme" json:"theme"`
 	// Custom CSS to be included when rendering this account's profile or statuses.
+	// Use empty string to unset.
 	CustomCSS *string `form:"custom_css" json:"custom_css"`
 	// Enable RSS feed of public toots for this account at /@[username]/feed.rss
 	EnableRSS *bool `form:"enable_rss" json:"enable_rss"`
diff --git a/internal/api/model/theme.go b/internal/api/model/theme.go
new file mode 100644
index 000000000..b98e7395a
--- /dev/null
+++ b/internal/api/model/theme.go
@@ -0,0 +1,32 @@
+// GoToSocial
+// Copyright (C) GoToSocial Authors admin@gotosocial.org
+// SPDX-License-Identifier: AGPL-3.0-or-later
+//
+// This program is free software: you can redistribute it and/or modify
+// it under the terms of the GNU Affero General Public License as published by
+// the Free Software Foundation, either version 3 of the License, or
+// (at your option) any later version.
+//
+// This program is distributed in the hope that it will be useful,
+// but WITHOUT ANY WARRANTY; without even the implied warranty of
+// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+// GNU Affero General Public License for more details.
+//
+// You should have received a copy of the GNU Affero General Public License
+// along with this program.  If not, see <http://www.gnu.org/licenses/>.
+
+package model
+
+// Theme represents one user-selectable preset CSS theme.
+//
+// swagger:model theme
+type Theme struct {
+	// User-facing title of this theme.
+	Title string `json:"title"`
+
+	// User-facing description of this theme.
+	Description string `json:"description"`
+
+	// FileName of this theme in the themes directory.
+	FileName string `json:"file_name"`
+}
diff --git a/internal/db/bundb/migrations/20240320114447_preset_css_themes.go b/internal/db/bundb/migrations/20240320114447_preset_css_themes.go
new file mode 100644
index 000000000..5d3b9f83a
--- /dev/null
+++ b/internal/db/bundb/migrations/20240320114447_preset_css_themes.go
@@ -0,0 +1,55 @@
+// GoToSocial
+// Copyright (C) GoToSocial Authors admin@gotosocial.org
+// SPDX-License-Identifier: AGPL-3.0-or-later
+//
+// This program is free software: you can redistribute it and/or modify
+// it under the terms of the GNU Affero General Public License as published by
+// the Free Software Foundation, either version 3 of the License, or
+// (at your option) any later version.
+//
+// This program is distributed in the hope that it will be useful,
+// but WITHOUT ANY WARRANTY; without even the implied warranty of
+// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+// GNU Affero General Public License for more details.
+//
+// You should have received a copy of the GNU Affero General Public License
+// along with this program.  If not, see <http://www.gnu.org/licenses/>.
+
+package migrations
+
+import (
+	"context"
+	"strings"
+
+	"github.com/uptrace/bun"
+)
+
+func init() {
+	up := func(ctx context.Context, db *bun.DB) error {
+		// Add theme to account settings table.
+		_, err := db.ExecContext(ctx,
+			"ALTER TABLE ? ADD COLUMN ? TEXT",
+			bun.Ident("account_settings"), bun.Ident("theme"),
+		)
+		if err != nil {
+			e := err.Error()
+			if !(strings.Contains(e, "already exists") ||
+				strings.Contains(e, "duplicate column name") ||
+				strings.Contains(e, "SQLSTATE 42701")) {
+				return err
+			}
+		}
+
+		return nil
+	}
+
+	down := func(ctx context.Context, db *bun.DB) error {
+		return db.RunInTx(ctx, nil, func(ctx context.Context, tx bun.Tx) error {
+			return nil
+		})
+	}
+
+	if err := Migrations.Register(up, down); err != nil {
+		panic(err)
+	}
+}
diff --git a/internal/gtsmodel/account.go b/internal/gtsmodel/account.go
index 2ac107e56..3bbcb37e3 100644
--- a/internal/gtsmodel/account.go
+++ b/internal/gtsmodel/account.go
@@ -220,3 +220,17 @@ type Relationship struct {
 	Endorsed            bool   // Are you featuring this user on your profile?
 	Note                string // Your note on this account.
 }
+
+// Theme represents a user-selected
+// CSS theme for an account.
+type Theme struct {
+	// User-facing title of this theme.
+	Title string
+
+	// User-facing description of this theme.
+	Description string
+
+	// FileName of this theme in the themes
+	// directory (eg., `light-blurple.css`).
+	FileName string
+}
diff --git a/internal/gtsmodel/accountsettings.go b/internal/gtsmodel/accountsettings.go
index cb5411050..218767023 100644
--- a/internal/gtsmodel/accountsettings.go
+++ b/internal/gtsmodel/accountsettings.go
@@ -29,6 +29,7 @@ type AccountSettings struct {
 	Sensitive         *bool      `bun:",nullzero,notnull,default:false"`                             // Set posts from this account to sensitive by default?
 	Language          string     `bun:",nullzero,notnull,default:'en'"`                              // What language does this account post in?
 	StatusContentType string     `bun:",nullzero"`                                                   // What is the default format for statuses posted by this account (only for local accounts).
+	Theme             string     `bun:",nullzero"`                                                   // Preset CSS theme filename selected by this Account (empty string if nothing set).
 	CustomCSS         string     `bun:",nullzero"`                                                   // Custom CSS that should be displayed for this Account's profile and statuses.
 	EnableRSS         *bool      `bun:",nullzero,notnull,default:false"`                             // enable RSS feed subscription for this account's public posts at [URL]/feed
 	HideCollections   *bool      `bun:",nullzero,notnull,default:false"`                             // Hide this account's followers/following collections.
diff --git a/internal/processing/account/account.go b/internal/processing/account/account.go
index 79f6ecfc1..dbcecdb0a 100644
--- a/internal/processing/account/account.go
+++ b/internal/processing/account/account.go
@@ -44,6 +44,7 @@ type Processor struct {
 	formatter    *text.Formatter
 	federator    *federation.Federator
 	parseMention gtsmodel.ParseMentionFunc
+	themes       *Themes
 }
 
 // New returns a new account processor.
@@ -67,5 +68,6 @@ func New(
 		formatter:    text.NewFormatter(state.DB),
 		federator:    federator,
 		parseMention: parseMention,
+		themes:       PopulateThemes(),
 	}
 }
diff --git a/internal/processing/account/themes.go b/internal/processing/account/themes.go
new file mode 100644
index 000000000..4f8cc49a1
--- /dev/null
+++ b/internal/processing/account/themes.go
@@ -0,0 +1,151 @@
+// GoToSocial
+// Copyright (C) GoToSocial Authors admin@gotosocial.org
+// SPDX-License-Identifier: AGPL-3.0-or-later
+//
+// This program is free software: you can redistribute it and/or modify
+// it under the terms of the GNU Affero General Public License as published by
+// the Free Software Foundation, either version 3 of the License, or
+// (at your option) any later version.
+//
+// This program is distributed in the hope that it will be useful,
+// but WITHOUT ANY WARRANTY; without even the implied warranty of
+// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+// GNU Affero General Public License for more details.
+//
+// You should have received a copy of the GNU Affero General Public License
+// along with this program.  If not, see <http://www.gnu.org/licenses/>.
+
+package account
+
+import (
+	"cmp"
+	"os"
+	"path/filepath"
+	"regexp"
+	"slices"
+	"strings"
+
+	"codeberg.org/gruf/go-bytesize"
+	apimodel "github.com/superseriousbusiness/gotosocial/internal/api/model"
+	"github.com/superseriousbusiness/gotosocial/internal/config"
+	"github.com/superseriousbusiness/gotosocial/internal/gtsmodel"
+	"github.com/superseriousbusiness/gotosocial/internal/log"
+)
+
+var (
+	themeTitleRegex       = regexp.MustCompile(`(?m)^\ *theme-title:(.*)$`)
+	themeDescriptionRegex = regexp.MustCompile(`(?m)^\ *theme-description:(.*)$`)
+)
+
+// GetThemes returns available account css themes.
+func (p *Processor) ThemesGet() []apimodel.Theme {
+	return p.converter.ThemesToAPIThemes(p.themes.SortedByTitle)
+}
+
+// Themes represents an in-memory
+// storage structure for themes.
+type Themes struct {
+	// Themes sorted alphabetically
+	// by title (case insensitive).
+	SortedByTitle []*gtsmodel.Theme
+
+	// ByFileName contains themes retrievable
+	// by their filename eg., `light-blurple.css`.
+	ByFileName map[string]*gtsmodel.Theme
+}
+
+// PopulateThemes parses available account CSS
+// themes from the web assets themes directory.
+func PopulateThemes() *Themes {
+	webAssetsAbsFilePath, err := filepath.Abs(config.GetWebAssetBaseDir())
+	if err != nil {
+		log.Panicf(nil, "error getting abs path for web assets: %v", err)
+	}
+
+	themesAbsFilePath := filepath.Join(webAssetsAbsFilePath, "themes")
+	themesFiles, err := os.ReadDir(themesAbsFilePath)
+	if err != nil {
+		log.Warnf(nil, "error reading themes at %s: %v", themesAbsFilePath, err)
+		return nil
+	}
+
+	themes := &Themes{
+		ByFileName: make(map[string]*gtsmodel.Theme),
+	}
+
+	for _, f := range themesFiles {
+		// Ignore nested directories.
+		if f.IsDir() {
+			continue
+		}
+
+		// Ignore weird files.
+		info, err := f.Info()
+		if err != nil {
+			continue
+		}
+
+		// Ignore really big files.
+		if info.Size() > int64(bytesize.MiB) {
+			continue
+		}
+
+		// Get just the name of the
+		// file, eg `blurple-light.css`.
+		fileName := f.Name()
+
+		// Get just the `.css` part.
+		extensionWithDot := filepath.Ext(fileName)
+
+		// Remove any leading `.`
+		extension := strings.TrimPrefix(extensionWithDot, ".")
+
+		// Ignore non-css files.
+		if extension != "css" {
+			continue
+		}
+
+		// Load the file contents.
+		path := filepath.Join(themesAbsFilePath, fileName)
+		contents, err := os.ReadFile(path)
+		if err != nil {
+			log.Warnf(nil, "error reading css theme at %s: %v", path, err)
+			continue
+		}
+
+		// Try to parse a title and description
+		// for this theme from the file itself.
+		var themeTitle string
+		titleMatches := themeTitleRegex.FindSubmatch(contents)
+		if len(titleMatches) == 2 {
+			themeTitle = strings.TrimSpace(string(titleMatches[1]))
+		} else {
+			// Fall back to file name
+			// without `.css` suffix.
+			themeTitle = strings.TrimSuffix(fileName, ".css")
+		}
+
+		var themeDescription string
+		descMatches := themeDescriptionRegex.FindSubmatch(contents)
+		if len(descMatches) == 2 {
+			themeDescription = strings.TrimSpace(string(descMatches[1]))
+		}
+
+		theme := &gtsmodel.Theme{
+			Title:       themeTitle,
+			Description: themeDescription,
+			FileName:    fileName,
+		}
+
+		themes.SortedByTitle = append(themes.SortedByTitle, theme)
+		themes.ByFileName[fileName] = theme
+	}
+
+	// Sort themes alphabetically
+	// by title (case insensitive).
+	slices.SortFunc(themes.SortedByTitle, func(a, b *gtsmodel.Theme) int {
+		return cmp.Compare(strings.ToLower(a.Title), strings.ToLower(b.Title))
+	})
+
+	return themes
+}
diff --git a/internal/processing/account/themes_test.go b/internal/processing/account/themes_test.go
new file mode 100644
index 000000000..9506aee50
--- /dev/null
+++ b/internal/processing/account/themes_test.go
@@ -0,0 +1,52 @@
+// GoToSocial
+// Copyright (C) GoToSocial Authors admin@gotosocial.org
+// SPDX-License-Identifier: AGPL-3.0-or-later
+//
+// This program is free software: you can redistribute it and/or modify
+// it under the terms of the GNU Affero General Public License as published by
+// the Free Software Foundation, either version 3 of the License, or
+// (at your option) any later version.
+//
+// This program is distributed in the hope that it will be useful,
+// but WITHOUT ANY WARRANTY; without even the implied warranty of
+// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+// GNU Affero General Public License for more details.
+//
+// You should have received a copy of the GNU Affero General Public License
+// along with this program.  If not, see <http://www.gnu.org/licenses/>.
+
+package account_test
+
+import (
+	"testing"
+
+	"github.com/stretchr/testify/suite"
+	"github.com/superseriousbusiness/gotosocial/internal/config"
+	"github.com/superseriousbusiness/gotosocial/internal/processing/account"
+)
+
+type ThemesTestSuite struct {
+	AccountStandardTestSuite
+}
+
+func (suite *ThemesTestSuite) TestPopulateThemes() {
+	config.SetWebAssetBaseDir("../../../web/assets")
+
+	themes := account.PopulateThemes()
+	if themes == nil {
+		suite.FailNow("themes was nil")
+	}
+
+	suite.NotEmpty(themes.SortedByTitle)
+	theme := themes.ByFileName["blurple-light.css"]
+	if theme == nil {
+		suite.FailNow("theme was nil")
+	}
+	suite.Equal("Blurple (light)", theme.Title)
+	suite.Equal("Official light blurple theme", theme.Description)
+	suite.Equal("blurple-light.css", theme.FileName)
+}
+
+func TestThemesTestSuite(t *testing.T) {
+	suite.Run(t, new(ThemesTestSuite))
+}
diff --git a/internal/processing/account/update.go b/internal/processing/account/update.go
index 7b5561138..076b6d7f4 100644
--- a/internal/processing/account/update.go
+++ b/internal/processing/account/update.go
@@ -256,6 +256,22 @@ func (p *Processor) Update(ctx context.Context, account *gtsmodel.Account, form
 		}
 	}
 
+	if form.Theme != nil {
+		theme := *form.Theme
+		if theme == "" {
+			// Empty is easy, just clear this.
+			account.Settings.Theme = ""
+		} else {
+			// Theme was provided, check
+			// against known available themes.
+			if _, ok := p.themes.ByFileName[theme]; !ok {
+				err := fmt.Errorf("theme %s not available on this instance, see /api/v1/accounts/themes for available themes", theme)
+				return nil, gtserror.NewErrorBadRequest(err, err.Error())
+			}
+			account.Settings.Theme = theme
+		}
+	}
+
 	if form.CustomCSS != nil {
 		customCSS := *form.CustomCSS
 		if err := validate.CustomCSS(customCSS); err != nil {
diff --git a/internal/typeutils/internaltofrontend.go b/internal/typeutils/internaltofrontend.go
index 0ff3c2268..bf44c7254 100644
--- a/internal/typeutils/internaltofrontend.go
+++ b/internal/typeutils/internaltofrontend.go
@@ -170,12 +170,13 @@ func (c *Converter) AccountToAPIAccountPublic(ctx context.Context, a *gtsmodel.A
 	// Bits that vary between remote + local accounts:
 	//   - Account (acct) string.
 	//   - Role.
-	//   - Settings things (enableRSS, customCSS).
+	//   - Settings things (enableRSS, theme, customCSS).
 
 	var (
 		acct      string
 		role      *apimodel.AccountRole
 		enableRSS bool
+		theme     string
 		customCSS string
 	)
 
@@ -208,6 +209,7 @@ func (c *Converter) AccountToAPIAccountPublic(ctx context.Context, a *gtsmodel.A
 			}
 
 			enableRSS = *a.Settings.EnableRSS
+			theme = a.Settings.Theme
 			customCSS = a.Settings.CustomCSS
 		}
 
@@ -272,6 +274,7 @@ func (c *Converter) AccountToAPIAccountPublic(ctx context.Context, a *gtsmodel.A
 		Emojis:         apiEmojis,
 		Fields:         fields,
 		Suspended:      !a.SuspendedAt.IsZero(),
+		Theme:          theme,
 		CustomCSS:      customCSS,
 		EnableRSS:      enableRSS,
 		Role:           role,
@@ -1771,3 +1774,16 @@ func (c *Converter) convertTagsToAPITags(ctx context.Context, tags []*gtsmodel.T
 
 	return apiTags, errs.Combine()
 }
+
+// ThemesToAPIThemes converts a slice of gtsmodel Themes into apimodel Themes.
+func (c *Converter) ThemesToAPIThemes(themes []*gtsmodel.Theme) []apimodel.Theme {
+	apiThemes := make([]apimodel.Theme, len(themes))
+	for i, theme := range themes {
+		apiThemes[i] = apimodel.Theme{
+			Title:       theme.Title,
+			Description: theme.Description,
+			FileName:    theme.FileName,
+		}
+	}
+	return apiThemes
+}
diff --git a/internal/web/profile.go b/internal/web/profile.go
index b629c98b9..a4809a72d 100644
--- a/internal/web/profile.go
+++ b/internal/web/profile.go
@@ -140,16 +140,40 @@ func (m *Module) profileGETHandler(c *gin.Context) {
 		return
 	}
 
+	// Prepare stylesheets for profile.
+	stylesheets := make([]string, 0, 6)
+
+	// Basic profile stylesheets.
+	stylesheets = append(
+		stylesheets,
+		[]string{
+			cssFA,
+			cssStatus,
+			cssThread,
+			cssProfile,
+		}...,
+	)
+
+	// User-selected theme if set.
+	if theme := targetAccount.Theme; theme != "" {
+		stylesheets = append(
+			stylesheets,
+			themesPathPrefix+"/"+theme,
+		)
+	}
+
+	// Custom CSS for this user last in cascade.
+	stylesheets = append(
+		stylesheets,
+		"/@"+targetAccount.Username+"/custom.css",
+	)
+
 	page := apiutil.WebPage{
-		Template: "profile.tmpl",
-		Instance: instance,
-		OGMeta:   apiutil.OGBase(instance).WithAccount(targetAccount),
-		Stylesheets: []string{
-			cssFA, cssStatus, cssThread, cssProfile,
-			// Custom CSS for this user last in cascade.
-			"/@" + targetAccount.Username + "/custom.css",
-		},
-		Javascript: []string{jsFrontend},
+		Template:    "profile.tmpl",
+		Instance:    instance,
+		OGMeta:      apiutil.OGBase(instance).WithAccount(targetAccount),
+		Stylesheets: stylesheets,
+		Javascript:  []string{jsFrontend},
 		Extra: map[string]any{
 			"account":          targetAccount,
 			"rssFeed":          rssFeed,
diff --git a/internal/web/thread.go b/internal/web/thread.go
index 4dcd1d221..ffec565e6 100644
--- a/internal/web/thread.go
+++ b/internal/web/thread.go
@@ -138,16 +138,39 @@ func (m *Module) threadGETHandler(c *gin.Context) {
 		return
 	}
 
+	// Prepare stylesheets for thread.
+	stylesheets := make([]string, 0, 5)
+
+	// Basic thread stylesheets.
+	stylesheets = append(
+		stylesheets,
+		[]string{
+			cssFA,
+			cssStatus,
+			cssThread,
+		}...,
+	)
+
+	// User-selected theme if set.
+	if theme := targetAccount.Theme; theme != "" {
+		stylesheets = append(
+			stylesheets,
+			themesPathPrefix+"/"+theme,
+		)
+	}
+
+	// Custom CSS for this user last in cascade.
+	stylesheets = append(
+		stylesheets,
+		"/@"+targetAccount.Username+"/custom.css",
+	)
+
 	page := apiutil.WebPage{
-		Template: "thread.tmpl",
-		Instance: instance,
-		OGMeta:   apiutil.OGBase(instance).WithStatus(status),
-		Stylesheets: []string{
-			cssFA, cssStatus, cssThread,
-			// Custom CSS for this user last in cascade.
-			"/@" + targetUsername + "/custom.css",
-		},
-		Javascript: []string{jsFrontend},
+		Template:    "thread.tmpl",
+		Instance:    instance,
+		OGMeta:      apiutil.OGBase(instance).WithStatus(status),
+		Stylesheets: stylesheets,
+		Javascript:  []string{jsFrontend},
 		Extra: map[string]any{
 			"status":  status,
 			"context": context,
diff --git a/internal/web/web.go b/internal/web/web.go
index 5e57d08c6..19df63332 100644
--- a/internal/web/web.go
+++ b/internal/web/web.go
@@ -44,6 +44,7 @@ const (
 	rssFeedPath        = profileGroupPath + "/feed.rss"
 	assetsPathPrefix   = "/assets"
 	distPathPrefix     = assetsPathPrefix + "/dist"
+	themesPathPrefix   = assetsPathPrefix + "/themes"
 	settingsPathPrefix = "/settings"
 	settingsPanelGlob  = settingsPathPrefix + "/*panel"
 	userPanelPath      = settingsPathPrefix + "/user"