Improve contrast for crosshair cursors

Summary:
The crosshair cursors had barely visible outlines, unlike their companions, so they were almost impossible to distinguish against backgrounds of a similar color. This patch fixes that.

BUG: 400110
FIXED-IN: 5.12.8

Test Plan:
Breeze crosshair against a dark background:

Breeze Snow crosshair against a light background:

Reviewers: Breeze, VDG, Plasma, ngraham

Reviewed By: Breeze, VDG, ngraham

Subscribers: ndavis, plasma-devel

Tags: Plasma

Differential Revision: https://phabricator.kde.org/D16861