Replace theme CSS with updated versions, add README with previews

Update theme-light.css and theme-dark.css to the latest GitHub-style
Gitea 1.27.x theme revisions, and add a README with preview images
and usage instructions.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_015g6ECVSKcBG8CJPrqC3G5o

README.md

@@ -1,4 +1,33 @@
 # css
-css foog
 
-<img src="assets/wMZ6FLJA-MZRb6z7k9CuB4CrYa_qkPuw9zYO0DsyeeltvQDykGb3Bug9mgroKkAp26-_vfK4owlyhKNUZWXS1bky.jpg">
+GitHub-style themes for **Gitea 1.27.x** — light and dark, built with modern CSS (`color-mix()`), matching GitHub's typography, spacing and color tokens.
+
+<p align="center">
+  <img src="assets/preview-2.jpg" width="100%" alt="Theme preview">
+</p>
+
+<p align="center">
+  <img src="assets/preview-1.png" width="160" alt="css">
+</p>
+
+## Themes
+
+| File | Description |
+|---|---|
+| [`css/theme-light.css`](css/theme-light.css) | GitHub Light — for `prefers-color-scheme: light` |
+| [`css/theme-dark.css`](css/theme-dark.css) | GitHub Dark — for `prefers-color-scheme: dark` |
+| [`css/theme-auto.css`](css/theme-auto.css) | Switches between the two automatically based on system preference |
+
+## Requirements
+
+A browser with `color-mix()` support: Chrome 111+, Firefox 113+, Safari 16.2+.
+
+## Installation
+
+1. Copy the files from [`css/`](css/) into your Gitea instance's `custom/public/assets/css/` directory (or wherever your Gitea deployment serves custom CSS from).
+2. Reference `theme-auto.css` (or `theme-light.css` / `theme-dark.css` directly) from your Gitea custom template, or set it as a selectable theme in `app.ini`.
+3. Restart Gitea, or reload the page, to pick up the new styles.
+
+## License
+
+Use and adapt freely.