feat(extensions): add support for custom themes in extensions (#17327)

packages/cli/src/commands/extensions/examples/themes-example/README.md

@@ -0,0 +1,28 @@
+# Themes Example
+
+This is an example of a Gemini CLI extension that adds a custom theme.
+
+## How to use
+
+1.  Link this extension:
+
+    ```bash
+    gemini extensions link packages/cli/src/commands/extensions/examples/themes-example
+    ```
+
+2.  Set the theme in your settings file (`~/.gemini/config.yaml`):
+
+    ```yaml
+    ui:
+      theme: 'shades-of-green-theme (themes-example)'
+    ```
+
+    Alternatively, you can set it through the UI by running `gemini` and then
+    typing `/theme` and pressing Enter.
+
+3.  **Observe the Changes:**
+
+    After setting the theme, you should see the changes reflected in the Gemini
+    CLI's UI. The background will be a dark green, the primary text a lighter
+    green, and various other UI elements will display different shades of green,
+    as defined in this extension's `gemini-extension.json` file.