This cleans up the CSS, reduces the number of different colors and special settings we use.
It increases contrast so we now pass WCAG AAA (according to chromium), and switches to css variables for colors to make dark mode simpler to implement.
(cherry picked from commit b48fa1f1a0)
For some reason I've seen one version of firefox use this over the one
we set in pydoctheme.css. Since we set it there in both light and dark
mode, this one should not be used.
Especially in dark-mode this was often too close to the background.
Should make it easier to read.
As always, colors not checked for artistic merit for I have none.
This led to pydoctheme.css being included *twice*, which led to
everything it included being included twice, which was annoying in
firefox when playing with the styles.
I don't *think* it had any performance impact?
