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.
The type hints allow mypy to check the gui_hook calls, revealing a
bunch of places that are broken as they expect no arguments like the
legacy hooks.
To make mypy happy about PyQt's signal.connect(func), a qconnect()
helper has been added.
