After reading Simple user styles I immediately downloaded Stylish (Chrome extension) and started messing around with Facebook. I always found UI filled with unnecessary stuff and I could finally remove certain parts (i.e. hide them).
Since I also like darker themes, quick result is something I call Darkbook Take a look:
I changed the default fonts, removed the right layout (who uses it anyway) and stretched the middle column a little bit to the right. I really like Google Web Fonts, you should check them out. I used Ubuntu for left column and Open Sans for everything else.
You can find the code on Github. Please note that this is merely an attempt at restyling, not a complete and usable style. I will continue to work on it (probably) and make it better over time.