Thank you for the initial links.
There are many hundreds of themes listed on there. Perhaps someone can go choose a few they think might be worth installing or using as a base for customization.
Installing a new theme looks straightforward enough. I'd still like to understand a little more about how they work and what security implications they may have. There are probably some upgrade implications too, but I have a feeling that isn't too bad.
If a theme is customized, the work should probably be done in a version control system. The initial checkin, if based on something downloaded, should exactly match the download package. That would make customizations stand out easily by doing a diff with the initial checkin. That could be important for forum upgrades. It'd also be nice from a security audit standpoint.
I can load the themes once they are chosen.