Updated Docs to inform users about the ability to have custom theme (#4632)

Co-authored-by: Jacob Richman <jacob314@gmail.com>

docs/cli/themes.md

@@ -32,6 +32,84 @@ Gemini CLI comes with a selection of pre-defined themes, which you can list usin
 
 Selected themes are saved in Gemini CLI's [configuration](./configuration.md) so your preference is remembered across sessions.
 
+---
+
+## Custom Color Themes
+
+Gemini CLI allows you to create your own custom color themes by specifying them in your `settings.json` file. This gives you full control over the color palette used in the CLI.
+
+### How to Define a Custom Theme
+
+Add a `customThemes` block to your user, project, or system `settings.json` file. Each custom theme is defined as an object with a unique name and a set of color keys. For example:
+
+```json
+{
+  "customThemes": {
+    "MyCustomTheme": {
+      "name": "MyCustomTheme",
+      "type": "custom",
+      "Background": "#181818",
+      "Foreground": "#F8F8F2",
+      "LightBlue": "#82AAFF",
+      "AccentBlue": "#61AFEF",
+      "AccentPurple": "#C678DD",
+      "AccentCyan": "#56B6C2",
+      "AccentGreen": "#98C379",
+      "AccentYellow": "#E5C07B",
+      "AccentRed": "#E06C75",
+      "Comment": "#5C6370",
+      "Gray": "#ABB2BF"
+    }
+  }
+}
+```
+
+**Color keys:**
+
+- `Background`
+- `Foreground`
+- `LightBlue`
+- `AccentBlue`
+- `AccentPurple`
+- `AccentCyan`
+- `AccentGreen`
+- `AccentYellow`
+- `AccentRed`
+- `Comment`
+- `Gray`
+
+**Required Properties:**
+
+- `name` (must match the key in the `customThemes` object and be a string)
+- `type` (must be the string `"custom"`)
+- `Background`
+- `Foreground`
+- `LightBlue`
+- `AccentBlue`
+- `AccentPurple`
+- `AccentCyan`
+- `AccentGreen`
+- `AccentYellow`
+- `AccentRed`
+- `Comment`
+- `Gray`
+
+You can use either hex codes (e.g., `#FF0000`) **or** standard CSS color names (e.g., `coral`, `teal`, `blue`) for any color value. See [CSS color names](https://developer.mozilla.org/en-US/docs/Web/CSS/color_value#color_keywords) for a full list of supported names.
+
+You can define multiple custom themes by adding more entries to the `customThemes` object.
+
+### Example Custom Theme
+
+<img src="../assets/theme-custom.png" alt="Custom theme example" width="600" />
+
+### Using Your Custom Theme
+
+- Select your custom theme using the `/theme` command in Gemini CLI. Your custom theme will appear in the theme selection dialog.
+- Or, set it as the default by adding `"theme": "MyCustomTheme"` to your `settings.json`.
+- Custom themes can be set at the user, project, or system level, and follow the same [configuration precedence](./configuration.md) as other settings.
+
+---
+
 ## Dark Themes
 
 ### ANSI