Supporting Mac OS X has turned out to be too difficult. I was able to fix the scrollbar issue but it’s a clumsy solution I’d rather not have to deal with. And even then, a host of other issues remain. So I will just consider it unsupported. If anyone wants to create Mac OS X versions of my themes, let me know.
I’ve marked the themes as for Windows on Firefox Add-ons. I’m not sure it’s necessary to mark other systems such as Linux unsupported, but they only allow the choice of a single platform or all. There appears to be many problems on Linux anyway, so it might be best. You can always download and install it manually.