Issue #7500: Define symbolic icon colors for System themes

This patch attempts to resolve #7500 (closed)

The layer tree icons use the GtkTreeView foreground color for symbolic icons. It appears that on System theme, this is commonly set to white - and so the icons aren't visible on the already white background.

To resolve this, I added a gimp-light.css to the /themes/System/ folder and defined the icon and header colors. It seems to fix the issue for me, but I'd appreciate thoughts from our resident theme experts @ajonsson, @brunolopesdsilv, and @rangelovd (and anyone else who's interested)!