When editing the styles for a specific block, it might be nice to offer an input for custom css. This way folks don't have to worry about finding / using the best selector.

Mockup: a "Custom CSS" button at the bottom of the Advanced section of a Group block, which slides the inspector to the right just as the content-only editing system, and Global Styles, does:

This could align well with how it could look for Global Styles:

Issue updated Oct 25. Related to #30142