Add Theme docs (#753)

docs/cli/themes.md

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+# Themes
+
+The Gemini CLI supports a variety of themes to customize its appearance. You can change the theme to suit your preferences via the `/theme` command. Below are the available themes:
+
+## Dark Themes
+
+### ANSI
+
+<img src="../assets/theme-ansi.png" alt="ANSI theme" width="600" />
+
+### Atom OneDark
+
+<img src="../assets/theme-atom-one.png" alt="Atom One theme" width="600">
+
+### Ayu
+
+<img src="../assets/theme-ayu.png" alt="Ayu theme" width="600">
+
+### Default
+
+<img src="../assets/theme-default.png" alt="Default theme" width="600">
+
+### Dracula
+
+<img src="../assets/theme-dracula.png" alt="Dracula theme" width="600">
+
+### GitHub
+
+<img src="../assets/theme-github.png" alt="GitHub theme" width="600">
+
+## Light Themes
+
+### ANSI Light
+
+<img src="../assets/theme-ansi-light.png" alt="ANSI Light theme" width="600">
+
+### Ayu Light
+
+<img src="../assets/theme-ayu-light.png" alt="Ayu Light theme" width="600">
+
+### Default Light
+
+<img src="../assets/theme-default-light.png" alt="Default Light theme" width="600">
+
+### GitHub Light
+
+<img src="../assets/theme-github-light.png" alt="GitHub Light theme" width="600">
+
+### Google Code
+
+<img src="../assets/theme-google-light.png" alt="Google Code theme" width="600">
+
+### Xcode
+
+<img src="../assets/theme-xcode-light.png" alt="Xcode Light theme" width="600">