add `custom_css` param to theme class (#12871)
* add custom css param to themeclass
* add changeset
* add gradio version
* add documentation
* fix app.py used by push to hub
* add changeset
* - check minor instead of patch
- add test
* lint
* add css var reference and generation script
* tweak custom_css example
---------
Co-authored-by: gradio-pr-bot <gradio-pr-bot@users.noreply.github.com>