Also some general improvements to material.css and solarized.css

Otherwise, you'd get a cursor/text misalignment in some fonts.

It's tiny anyway, no need to force people to include yet another file.

Signed-off-by: Jan Keromnes <janx@linux.com>

This normalizes the classes used by most of the modes
(reStructuredText and diff didn't fit in), and makes them all
reference a central style sheet (under theme/). It also adds a theme
option that can be set to change the theme, and defines a few
different themes (some based on old mode colors).