kstyle: make MDI title buttons legible

Don't blend window background (default white) with selected text color
(also white), as this results in unreadable combinations. Instead, blend
it with the normal color.

BUG: 438964
FIXED-IN: 5.24.2

(cherry picked from commit 6d3c04d7)
2 jobs for Plasma/5.24 in 2 minutes and 16 seconds (queued for 12 seconds)