This replaces the previous CSS theme system with a new one based around
using SCSS map-merge to define different "color sets" that can overwrite
the base theme to generate new themes.
The original theme system was heavily based around Solarized with only
light and dark variants, and other themes weren't able to replace many
of the colors (link colors, buttons, alerts, etc.). This new system will
make it possible to have themes that are completely unrelated to the
Solarized variants.
Bauke
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