you can create custom textures, but they stretch weird in different views. right click on renoise, select show package contents > contents > resources > skin > textures. duplicate one of the texture folders and rename it, then open a .bmp editor and have at it. BodyTexture.bmp and InnerBodyTexture.bmp is the place to start, I believe. Preserve the names when saving. save format and color formatting may well make a difference. good luck!
there’s a texture thread here: More GUI Textures (Please)