docs: Update README, CONTRIBUTING, and CLI configuration docs (#591)

cleaning up for now, will take another pass

docs/cli/configuration.md

@@ -222,3 +222,36 @@ This example demonstrates how you can provide general project context, specific
   - See the [Commands documentation](./commands.md#memory) for full details on the `/memory` command and its sub-commands (`show` and `refresh`).
 
 By understanding and utilizing these configuration layers and the hierarchical nature of `GEMINI.md` files, you can effectively manage the AI's memory and tailor the Gemini CLI's responses to your specific needs and projects.
+
+## Theming
+
+The Gemini CLI supports theming to customize its color scheme and appearance. Themes define colors for text, backgrounds, syntax highlighting, and other UI elements.
+
+### Available Themes
+
+The CLI comes with a selection of pre-defined themes. As seen in `theme-manager.ts`, these typically include:
+
+- **Dark Themes:**
+  - `AtomOneDark`
+  - `Dracula`
+  - `VS2015` (Default)
+  - `GitHub` (Dark variant usually)
+- **Light Themes:**
+  - `VS` (Visual Studio Light)
+  - `GoogleCode`
+  - `XCode` (Light variant usually)
+- **ANSI:**
+  - `ANSI`: A theme that primarily uses the terminal's native ANSI color capabilities.
+
+_(The exact list and their appearance can be confirmed by running the `/theme` command within the CLI.)_
+
+### Changing Themes
+
+1.  Type the `/theme` command in the CLI.
+2.  A dialog or selection prompt (`ThemeDialog.tsx`) will appear, listing the available themes.
+3.  You can typically navigate (e.g., with arrow keys) and select a theme. Some interfaces might offer a live preview or highlight as you select.
+4.  Confirm your selection (often with Enter) to apply the theme. You can usually cancel out of the selection (e.g., with Escape).
+
+### Theme Persistence
+
+Selected themes are usually saved in the CLI's configuration (see [CLI Configuration](./docs/cli/configuration.md)) so your preference is remembered across sessions.