Flavored-hreader is a project mainly written in JavaScript, it's free.
A customized helvetireader for Google Reader (with lots of eye candy!)
IMPORTANT: I am no longer maintaining or updating this extension. Instead, I am using Stylebot for applying the CSS in this extension.
Most recent versions of CSS:
Day Theme: http://stylebot.me/styles/61
Night Theme: http://stylebot.me/styles/62
Post your issues or tweaks on Stylebot Social: http://stylebot.me/styles/61