[Breeze Plasma Theme] Fix kleopatra icon to use color stylesheet

Summary:
Had hardcoded colors before.

BUG: 394400

Test Plan:
Looks properly in cuttlefish in black/white mode, both inverted &
non-inverted.

Reviewers: Plasma, aheinecke

Reviewed By: aheinecke

Subscribers: kde-frameworks-devel

Tags: Frameworks

Differential Revision: https://phabricator.kde.org/D12959