Gitea supports three official themes right now, gitea (light), arc-green (dark), and auto (automatically switches between the previous two depending on operating system settings). To add your own theme, currently the only way is to provide a complete theme (not just color overrides)
As an example, let’s say our theme is arc-blue (this is a real theme, and can be found in this issue)
Name the .css file theme-arc-blue.css and add it to your custom folder in custom/public/css
Allow users to use it by adding arc-blue to the list of THEMES in your app.ini
