It seems that a few important google apps already offer theme customization. Is there any way to change the theme on Google Reader as well?
No it is not possible to change the layout only through user scripts or Reader Play.
The most promising one for me is Helvetireader
You could style at least the sidebar for this session with a ninja design if you press ↑ ↑ ↓ ↓ ← → ← → B A.
This Firefox extension is pretty good. It offers a few different themes, plus several other enhancements.
Feedly is not a theme but an alternate view of your GReader, accessible through a Firefox or Chrome plugin
There's also Re-Reader, a bookmarklet which restyles the web interface giving you a more simplistic look.
I use the sublime reader extension for Google chrome. Comes with two themes, dark and light!
like this pretty much, very modern and nerdy grey look