We use unsaturated selection style for sidebar rows to make it grab  
less attention. Since we use the light variant of Adwaita, we do this 
by making the colors darker using the shade() CSS color	expression.

However, this assumption breaks	if the dark variant is explicitly set    
in Tweaks.  

While nautilus is not developed for the dark variant, this commit      
makes it work by deriving shading from other theme colors instead.

Fixes GNOME/nautilus#630