Seperate admin themes into CSS bundles

With seperate theme files for light, dark and auto modes we are able to load the theme
we want into other admins (ie. Solidus) while still keep the bundles as small as possible.

The default `alchemy/theme.css` file includes both (dark and light) themes which
switch by preferred color scheme.

The dedicated `alchemy/light-theme` and `alchemy/dark-theme` can be loaded in situations
where you want that explicit theme.

Author: tvdeyen