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.
I tried this hack, but i choose the last color  instead of the first color  the 12th color is red, and it is kinda neat when you choose that color to make black. each color background has a matching font shadow for that color. when red is replaced with a black background the text is surrounded by a red haze. quite nice. try it with other colors of your choice. [:
Thanks Cybojanek, that was a nice trick. I used the Silver one and reduced the brightness by (-)125 in Paint Shop Pro; that way you still get the original image (instead of just a solid black background). It looks great and makes it easier for me to know whether my PSP is in 1.5 or 2.71.