css: Support for dark mode

css: Support for dark mode

Modern browsers have a "dark mode" preference, which enables alternate
styles on web sites that support this.

This patch adds a dark color scheme, that is automatically activated
via a CSS @media query.

Older browsers that do not support color schemes will simply show the
light scheme, but possibly without syntax highlighting.

Note that filters that use color (such as source highlighters) and
logotypes may need to be updated to work with a black background!
See the updated files in the filters/ directory.

Signed-off-by: Samuel Lidén Borell <samuel@kodafritt.se>
Signed-off-by: Jason A. Donenfeld <Jason@zx2c4.com>