Unbreak icon coloring on Breeze, and remove dark icon theme

This icon theme is useless, because we only support Breeze Icons (and there is no longer a separate "Breeze Dark" icon theme). Also fix Breeze icons not recoloring properly, and leave better comments on the current workarounds.

The ultimate solution is to upstream our icons, but this should be a holdover in case I don't get to it in time.

When testing, please make sure to wipe your icon cache (~/.cache/icon-cache.kcache).