@markc: push your theme to GitHub and post the link. Maybe I can help because I can not really follow your problem description
@till: What you describe is exactly what GitHub is good for. You define a upstream in the forked repo (of the contributor) and make your fix/new library/whatever and push it to your GitHub account.
* If the changes are minor or can be confirmed to work visually you send a "pull request" -> core dev can comment on that and decline or merge the changes with one click on GitHub
* If it is somewhat bigger of a change you can test it while development continues. You test and rebase your fork continuously with the development branch of the upstream (in this example the master branch of ISPConfig)
This way changes by "untrusted" contributors are always visually checkable by everyone (I often comment on commits or pull request to point out mistakes or possible enhancements) and can be changed by the contributor in this process.
I know you do not like the discussion about GitHub and I would understand if you stop adding to it
about the theme: can push it to the SVN on completion although I do see a theme (since the theme switch was integrated) for ISPC as a standalone package. Maybe you could add it to the official apps & addon repo