It's become increasingly obvious that due to the misleading title of THIS thread, I will never get an answer there. So, I will describe the obvious bug here. Everytime I change my theme to black, since my GDNet+ expired, it goes back to the current default theme after I leave and come back, post, or do pretty much anything. I did not have this issue before I purchased GDNet+ or while I still had it. And, since others in that thread claimed to experience the same issue, I'd say it's obviously a bug.

This does indeed appear to be fixed. =)