Lighten button color for Breeze Light color scheme

The button coloration agreed to in the Breeze Evolution work wound up reverted in c4f93a4b because we were previously tinting the button color in the QStyle rather than changing the color scheme colors instead. The effect works for the dark color scheme, but not the light one, so we need to manually lighten the button color a bit there. This commit does that, restoring the intended button coloration for the Breeze Light color schemes as per the ongoing Breeze Evolution work.

Before	After

cc @teams/vdg @ndavis