add a facelift to dartdoc

[devoncarew]

We should borrow some of the new css look from dartlang.org.

[devoncarew]

@jcollins-g, here's the current look - the header's now a fixed size:

you can compare with a page like https://www.dartlang.org/community.

[jcollins-g]

looks very good!

[devoncarew]

The last bit left of this issue is to re-do the columns to be more similar to dartlang.org; this will improve the scrolling and nav behavior.