Custom Theme

Hi there,
I was thinking to create a PR for a new theme which matches the github default dark theme, then I realize that it would be much better if custom theme option would be there.
Like in url itself as a params user passes back-ground color, text-color, and content color similar to the custom font. Then selecting a theme will be more flexible.

The issue names no files, tests, or entry points. Start by locating the existing custom-font URL parameter handling and theme generation, then determine where background, text, and content colors are applied. Done means users can pass those color values in the URL and the generated card reflects them consistently.

Written by the indexing model from the issue text.