you can go back, but you would lose all the new 4.2 stuff. Generally when I want to go back i download the software I want to go back to, install it and delete the folder for the old OS and the vendor file. Then hook up the BB to DM and let it roll.
What theme are you looking for? There are a variety of themes on the BB by default, you might check under options themes for what you are looking for before giving up on 4.2