I’m hoping that both the Narrated Web Site Report maintained by @SNoiraud and the Themes Addon by @prculley will be extended to promote custom theme development. Adding a custom overlay theme selection (in addition to the existing baseline theme selection) would be ideal.
As @DaveSch suggests, Custom CSS could be stored in the User Data CSS folder by default.
My current overlay for (the Gramps GUI) has to point the cascade at the baseline CSS before my just my custom elements. (Currently, that just includes: the height of progress bars, and; toning down the Addon Manager’s lozenges.)
This unfortunately means that the baseline theme (and maybe the light/dark mode selection? I don’t recall) is hard-coded.