This change makes sure that the "UI coloring" feature respects the
original brightness of the current theme. It prevents dark themes from
getting a too light recoloring and vice versa.

Extra benefit: this allows to remove much recently introduced code.

Change-Id: Ib2c96e7ed172a4cc97520aa4b5d180cc6353c661
Reviewed-by: hjk <hjk@theqtcompany.com>