[TASK] Style buttons / invert (UX Vision)

Styles all buttons in the backend similar to UX vision paper.
Inverts buttons colors and changes font-family.

This patch does not bring styling to primary button: orange
or secondary button: silver.

Change-Id: I273e76a02083aedca4c6759948172b418e990ede
Resolves: #55592
Releases: 6.2
Reviewed-on: https://review.typo3.org/27265
Reviewed-by: Christian Kuhn
Tested-by: Christian Kuhn
Reviewed-by: Marcin Sągol
Reviewed-by: Georg Ringer
Tested-by: Georg Ringer