Allows add-ons to easily override the theme, and allows us to apply
styling to elements outside of the normal document flow (like applying
the normal background colour to a position: fixed element).
Forces the Fusion theme when running night mode, so we don't need
to work around platform themes that don't respond to the defined
palette.
Feedback/suggestions on the chosen colours welcome - _vars.scss is the
file to change if you want to experiment with adjustments.
