GTK3 - Fix menu port to colordefs

Summary: In D22877, some git spaghetti caused some work on _menus.scss to be lost.

Test Plan: See that menus look and behave as intended.

Reviewers: Plasma, VDG, ngraham

Reviewed By: VDG, ngraham

Subscribers: plasma-devel

Tags: Plasma

Differential Revision: https://phabricator.kde.org/D22971