Revert "[TASK] Style buttons / invert (UX Vision)"

This reverts commit de4811fc

This is not in line with the rest of the backend yet.
The "UX vision" is for after 6.2.

Change-Id: Iab8880163b0cb668237617b1260b4126b16d1cb3
Reviewed-on: https://review.typo3.org/27306
Reviewed-by: Ernesto Baschny
Tested-by: Ernesto Baschny