4chan's two new themes - Yotsuba / Yotsuba Blue gives us some nice updates to the layout. However, it fails because it forces the font and font size on the page instead of using browser defaults. Also, currently (2008-03-19) using the new themes breaks the 4chan extension button coloring.

This userstyle is meant to be used with the default Futaba / Burichan themes, and it takes most of the visual improvements of the new Yotsuba / Yotsuba Blue themes.

Update (2009-01-27): Floating navbar has been moved to its own style:
4chan - Floating navbar

Regarding the "after" screenshot - Helvetica is not included in the style. It uses your browser's default font(s).