[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

1 files changed, 30 insertions, 0 deletions

diff --git a/docs/admin/themes.md b/docs/admin/themes.md
new file mode 100644
index 000000000..a3ef11b05
--- /dev/null
+++ b/docs/admin/themes.md
@@ -0,0 +1,30 @@
+# Themes
+
+Users on your instance can select a theme for their profile from any css files present in the `web/assets/themes` directory.
+
+GoToSocial comes with some theme files already, but you can add more yourself by doing the following:
+
+1. Create a file in `web/assets/themes` called (for example) `new-theme.css`.
+2. (Optional) Include the following comment at the top of your theme file to title and describe your theme:
+  ```css
+  /*
+    theme-title: My New Theme
+    theme-description: This is an example theme
+  */
+  ```
+  You can use any text you like for these fields, but bear in mind whatever you write here will appear in the settings panel to help users when selecting a theme, so keep it short and sweet.
+3. Fill out your custom CSS in the rest of the file. You can use one of the existing CSS files to guide you. Also see [this page](../user_guide/custom_css.md) for some rough guidelines about how to write accessible CSS.
+4. Restart your instance so that the new CSS file is picked up.
+
+!!! info
+    If you're using Docker for your deployment, you can mount theme files from the host machine into your GoToSocial `web/assets/themes` directory instead, by including entries for them in the `volumes` section of your Docker configuration.
+    
+    For example, say you've created a theme on your host machine at `~/gotosocial/my-themes/new-theme.css`, you could mount that theme into the GoToSocial Docker container in the following way:
+    
+    ```yaml
+    volumes:
+      [.... some other volume entries ...]
+      - "~/gotosocial/my-themes/new-theme.css:/gotosocial/web/assets/themes/new-theme.css"
+    ```
+    
+    Bear in mind if you mount an entire directory to `/gotosocial/web/assets/themes` instead of mounting individual theme files, you'll override the default themes.