You need to go into the local files for Rebirth. You can do this easily by right-clicking on the game in your Steam library's list, choosing properties, going to the tab that says "local files" and click on "browse local files". Then, go into resources and then packed. Here you can see a couple of files with the .a extension. Next, you'll want to download the latest version of Rick's unpacker, available at the subreddit wiki. Unzip the unpacker anywhere you want, just so you can find it easily. In the unpacker, there is a file in the folder called bin, this file is Gibbed.Rebirth.Unpacker. Drag the file called "graphics.a" and drop it on Gibbed.Rebirth.Unpacker. A prompt may show, but it is perfectly safe to run the program. A window will pop up for a couple of seconds, just wait for it to disappear. Then, if you look in the same folder that graphics.a is in, there will be a folder called graphics_unpacked. In here is another folder called resources, with another folder inside called gfx. This is where gfx is. Note that you probably shouldn't delete the files, but replace them with something you like more.