Maintaining themes is burdensome, and the authors of new themes tend to disappear and themes are left unmaintained.
Principally the old "openwrt", current default "bootstrap" and new "openwrt-2020" are supported as official themes.
Additional themes in repo:
- "material" is in the repo, but pretty much unmaintained. In danger to fall into incompatiblity.
- "rosy" was marked BROKEN about one year after taking it into the repo, because the author did not maintain it.
Themes include some of the real functionality, and the layout CSS files need modifications after the underlying LuCI changes, so the authors would need to take care of the required changes.
So, introducing new themes to the official repo is unlikely.