TYPO3 Core

Reason for revert:

• Invasive change that was merged too quickly
• Missing commit message description * No description of the global scope/direction.
  Will the new style be applied to all backend
  interfaces? And who will feel responsible? * Deserves an epic on forge
• Missing credits to structured content initiative
• Quality issues: * Delete button removal does not have broad acceptance * Inconsistent border radii for content elements and grid cells * Inconsistent elevation (box-shadow) for elements and grid cells