Tweaking fonts.

I don't like Verdana here, so just use the default font if Lucida Grande
is unavailable.

Only use a serif for the heading of the post, not for subheadings.

Tweaking font sizes to be a large, readable size, without having too
many horizontal scroll bars on code blocks. We also avoid using relative
sizes, they're trickier to reason about.
Simplifying sidebar.