I thought I'd answer this question from a noob point of view (meaning some people on here are too smart and think everyone else here to be just as smart... therefore they they either talk over your head, avoid giving you a straight answer, or possibly don't realize we don't get it. Any way they do it their answers go over the noobs head).
So to answer your question. The "current build" rockbox offered here is a stable build, but it's kinda like your fathers car, it can be boring. Some people take that stable build and add unsupported
patches to it to allow it to show album art or scrolling text or many other bells and whistles. If the theme you download needs any of these bells and whistles (it will tell you what you need in the description) and you aren't using a build that has that/those patches then you will end up with the default screen. What you'll need is a custom build, read the description of the theme you're downloading to get an idea of what that build will need to run that theme.