[GTK3 -> Chrome/ium] Tweak tab stylings

Summary:
Tabs have had their colors changed to make it easier to tell which is active.

BUG: 403108

Test Plan:
Breeze Normal:
Breeze Dark:
Breeze Light:

Reviewers: Breeze, VDG, ngraham, GB_2

Reviewed By: Breeze, VDG, ngraham, GB_2

Subscribers: davidre, broulik, ngraham, plasma-devel

Tags: Plasma

Differential Revision: https://phabricator.kde.org/D23080