PieFed is unfortunately pretty poorly documented. There's a brief overview of theming that seems to be directed at people who don't know CSS, but that's pretty much it. If you want to create your own theme, their recommendation is to just...inspect whatever element you want to change and then start a stylesheet from scratch. Which is completely bizarre compared to any other similar software I've ever used, which always includes some kind of "default" theme you can tweak to your liking.