If you use Devhook (anyone that supports 2.71 emulation) you can have a black wavy background.
1. Open up a specific folder in your dump (E:/DH/271/flash0/vsh/resource).
2. There you will see 12 bmp files and some other ones.
3. Take 01.BMP and rename it to (for example 13.BMP)
4. Open it, change the color to black (for example in paint), without changing the resolution and then save it as 01.BMP. (In case you want to redo it back to silver).
5. When you boot up your PSP go to Settings and Themes. There, instead of a silver rectangle as the first one, you will have a black rectangle.
Select it and you will have a black backgound with the wavy lines.
You can basicly do this with any BMP, but I only tried it with the black one.