I think the issue in the link above might have been resolved but I think there's another issue with themes at the moment.
I've checked the themes that have been removed from the Rockbox theme page listing, and done a bit of testing with them, and I've deduced that the common factor is that they all have a backdrop on bar tags such as the progress or volume bars. If I try removing that code from my themes I can reupload them, but then they don't look right. I've tried reporting the issue to Rockbox but I'm not sure if or when it will get resolved.
On the one hand I'm really impressed with Rockbox but on the other I'm a bit unimpressed with the stability of the skin engine and theming.