Fix "Add Network Folder" tooltip icon does not show on Breeze, shows correctly on Breeze Dark

Summary:
Fix "Add Network Folder" tooltip icon does not show on Breeze, shows correctly
on Breeze Dark. The fix is taken from D19596.

Test Plan:
Open Dolphin with the Breeze theme
Hover the mouse over the "Add Network Folder"
The icon is black on black

BUG: 404858

Reviewers: #dolphin, elvisangelaccio, ngraham

Reviewed By: #dolphin, elvisangelaccio, ngraham

Subscribers: broulik, kfm-devel

Tags: #dolphin

Differential Revision: https://phabricator.kde.org/D19738