update readme

README.md

@@ -10,6 +10,7 @@ releases yet. For an example of a website built with Zona, please see
 [ficd.ca](https://ficd.ca)
 
 <!--toc:start-->
+
 - [Features](#features)
   - [Internal Link Resolution](#internal-link-resolution)
   - [Syntax Highlighting](#syntax-highlighting)
@@ -19,6 +20,7 @@ releases yet. For an example of a website built with Zona, please see
   - [Site Layout](#site-layout)
   - [Frontmatter](#frontmatter)
   - [Configuration](#configuration)
+
 <!--toc:end-->
 
 ## Features
@@ -54,7 +56,7 @@ including those pointing to static resources like images.
 ### Syntax Highlighting
 
 Zona uses [Pygments] to provide syntax highlighting for fenced code blocks. The
-following plugins are included:
+following Pygments plugins are included:
 
 - [pygments-kakoune](https://git.sr.ht/~ficd/pygments-kakoune)
   - A lexer providing for highlighting Kakoune code. Available under the `kak`
@@ -63,6 +65,23 @@ following plugins are included:
   - An implementation of the [Ashen](https://git.sr.ht/~ficd/ashen) theme for
     Pygments.
 
+If you want to use any external Pygments styles or lexers, they must be available
+in Zona's Python environment. For example, you can give Zona access to [Catppucin](https://github.com/catppuccin/python):
+
+```yaml
+# config.yml
+markdown:
+  syntax_highlighting:
+    theme: catppucin-mocha
+```
+
+Then, run Zona with the following `uv` command:
+
+```sh
+uvx --with catppucin zona build
+```
+
+
 ### Image Labels
 
 A feature unique to Zona is **image labels**. They make it easy to annotate