Use more correct color for close icons
This implements part of teams/vdg/issues#43 (closed). It is a less controversial alternative to !349
Right now these icons are red, which is incorrect since closing windows and tabs isn't an inherently destructive action. Fix this by making them black.
Here's how it will look with this MR:
Drawbacks:
- Kind of a heavy appearance
- Doesn't match the window decoration close button icon, but changing that is under discussion in plasma/breeze!438 (comment 911680).
Edited by Nate Graham