Eh....there are only four fixed folders, so you'd have to replace the content of one of them. It's been one of the first things I tried to optimize, but only ended up adding one custom folder. Later on I'll see if I can figure out some nicer windows routines to automatically detect new folders and content.
So for now just backup the custom folder, for example, and replace the original images. The palette image allows you to change the colors, but it's a bit tricky as you will find out. Many colors are being used in multiple places, even for effect purposes. I've changed it a little bit from the original to avoid some of the uglier side effects as they used to be. But that's mostly just for the mixer. All other synths will react a bit more sensitively...you'll see.