Revert 4d4db0b1's theme changes

Undoes the theme changes in that commit since they got pushed too
early in the Frameworks release cycle to be effectively Plasma 5.22
exclusive, saddling users on 5.21 with increased transparency all the
time with no way to turn it off. This was not the intended user
experience.

This commit will be reverted in time to be released with Frameworks 5.83,
which will align with the release of Plasma 5.22.

BUG: 434202