4 files changed, 221 insertions, 0 deletions
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 {