[TASK] Follow-up: Buttons CSS definition

Moves definitions to CSS/structure.
Also fixes right margin for a.t3-button.

Change-Id: Ib51e8b7e3c11e0a4cf474b6322aaeaed11c63b55
Resolves: #55588
Related: #55539
Releases: 6.2
Reviewed-on: https://review.typo3.org/27260
Reviewed-by: Marcin Sągol
Tested-by: Marcin Sągol
Reviewed-by: Wouter Wolters
Tested-by: Wouter Wolters