Every week (although this may be longer as we start back up again) the admin will post a theme, complete with pictures. You vote on the one you prefer, and the winners move on. It's really that simple - vote, and enjoy the pretty!
[eta] There are some older posts from when DG (formerly CW) and MW were done differently; we've chosen to leave those up for now, because baleeting is sort of an effort and eh. For now, however, those posts will still remain flocked, but all new posts will be public, as all voting will be.