Fix colors from KStatefulBrushes not using application colorschemes

Summary:
This fixes the colors of things like context menus and application menus

T11124#193132

Test Plan:
Old:

New:

Reviewers: VDG, Breeze

Subscribers: hpereiradacosta, mglb, ngraham, plasma-devel

Tags: Plasma

Differential Revision: https://phabricator.kde.org/D23170