Do you mean the background with the planets? You could try using a program like Tile Layer Pro (in Genesis format) to maybe track down potential background tiles and dump them to a bitmap for editing. You can also reinsert them. There are probably problems with this, because I remember hearing somewhere that lots of the SegaCD's bios data is compressed, and decompressed into memory while it's in use. Also you would likely need to modify the palette to accomodate any new colors you would introduce... please, someone smarter than me, help out!
Well if you _really_ want to modify it, you could try tracing the code somehow to find out how it decompresses it... and figure out some way to recompress it. You'd probably have to write a custom util for that. I don't think there is a way to trace 68k code in an emu or anything though.