Document usage of custom themes

Author: Naseschwarz

THEMES.md

@@ -3,6 +3,8 @@
 default on light terminal:
 ![](assets/light-theme.png)
 
+## Configuration
+
 To change the colors of the default theme you need to add a `theme.ron` file that contains the colors you want to override. Note that you don’t have to specify the full theme anymore (as of 0.23). Instead, it is sufficient to override just the values that you want to differ from their default values.
 
 The file uses the [Ron format](https://github.com/ron-rs/ron) and is located at one of the following paths, depending on your operating system:
@@ -31,14 +33,27 @@ Notes:
 * using a color like `yellow` might appear in whatever your terminal/theme defines for `yellow`
 * valid colors can be found in ratatui's [Color](https://docs.rs/ratatui/latest/ratatui/style/enum.Color.html) struct.
 * all customizable theme elements can be found in [`style.rs` in the `impl Default for Theme` block](https://github.com/gitui-org/gitui/blob/master/src/ui/style.rs#L305)
-* the syntax highlighting theme can be defined using the element `syntax`. Currently, only [default themes of the syntect library are supported](https://github.com/trishume/syntect/blob/7fe13c0fd53cdfa0f9fea1aa14c5ba37f81d8b71/src/dumps.rs#L215).
+
+## Syntax Highlighting
+
+The syntax highlighting theme can be defined using the element `syntax`. Both [default themes of the syntect library are supported](https://github.com/trishume/syntect/blob/7fe13c0fd53cdfa0f9fea1aa14c5ba37f81d8b71/src/dumps.rs#L215) and custom themes are supported.
 
 Example syntax theme:
 ```
 (
     syntax: Some("InspiredGitHub"),
 )
 ```
+
+Custom themes are located in the [configuration directory](#configuration), are using TextMate's theme format and must have a `.tmTheme` file extension. To load a custom theme, `syntax` must be set to the file name without the file extension. For example, to load [`Blackboard.tmTheme`](https://raw.githubusercontent.com/filmgirl/TextMate-Themes/refs/heads/master/Blackboard.tmTheme), place the file next to `theme.ron` and set:
+```
+(
+    syntax: Some("Blackboard"),
+)
+```
+
+[filmgirl/TextMate-Themes](https://github.com/filmgirl/TextMate-Themes) offers many [beautiful](https://inkdeep.github.io/TextMate-Themes) TextMate themes from which to choose.
+
 ## Customizing line breaks
 
 If you want to change how the line break is displayed in the diff, you can also specify `line_break` in your `theme.ron`: