Created and tested with Gitea v1.15. The theme might work with future versions though Gitea devs might change some CSS classes in the meantime and things might potentially look odd - just try yourself :)

I might update the theme over time to fix oversights and other issues - no guarantee though.

Installation

If you do not have admin access to a Gitea instance, you can use the Stylus browser extension and use the CSS provided in this repo for the Gitea URL
If you are an admin and want to make this theme available to all users:
1. Put theme-github.css into $GITEA_PUBLIC/public/css/theme-github.css where $GITEA_PUBLIC is the "CustomPath" of your instance reported by gitea help.
2. Add github to the comma-separated list in the setting THEMES in app.ini
3. Now users can select this theme in their settings under "account"
4. (optional) If you want to make this theme the default of your instance, set it in DEFAULT_THEME in app.ini

Changes to default Gitea theme

Aligned primary green, grey, red and blue colors with GitHub
Aligned border-radius with GitHub
Related theme by removing a lot of hover colors and dominant button background-colors
Aligned markup background to GitHub's value

Screenshots