tweak formatting of headers (esp. on long pages)?

[drewfish]

On long pages (such as the guide) the header structure sometimes gets down to <h4>. With our current formatting that isn't particularly distinguished from surrounding prose.

We could perhaps tweak the headers to be more visually distinct, such as with underlines or background colors.

Hmmm... if we have a table of contents (#84) perhaps this won't be such an issue.