Removed some big sections that seemed out of date, please call
me out on anything that seems too radical.

I didn't change the obviously out of date widget coloring section
because I'm curious what to replace it with: remove it completely and
point to HIG? Go to CSS theming and remove the things about RC files?

The gdk_cairo_create stuff is a bit useless nowadays, there's "how can
I create a cairo context" and then stuff about why you shouldn't do it
right under it. This is a GTK+ manual, so maybe the stuff about the
DRAW signal may be more appropriate instead.