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.