I've made a tutorial on installing Rockbox on a 5th generation iPod before, but you can visit Rockbox's website for a more up-to-date and detailed installation instructions for your specific iPod model.
I'm going to show to you some of the coolest user-uploaded Rockbox iPod themes that you can download for free. These are only for 5th generation iPod Video, but there are also available themes for other models as well. But first, let me show the default iPod 5G theme:
Now, feast your eyes on these rockin' Rockbox themes for iPod:
2. Molten Cobalt
6. jBlackGlass Themes
11. Theme X HighBar
15. iPod Green5g