Alter rust.css to make the documentation look more appealing

I was making documentation for my own little Rust project, and I was somewhat unhappy with how the documentation looked. While many of the issues are endemic to how rustdoc generates its output, you can get pretty far in making the documentation readable by using a better CSS style.

This commit alters the CSS style used in Rust's documentation in order to make the various sections stand out more. You can see an example of its usage in my own project's documentation: http://siegelord.github.io/RustGnuplot/#implementation-for-figureself-where-self. I showed it to some people on IRC and they suggested that I make a pull request here. I tested it on the only browser that matters, but also Chrome and Opera.