[TASK] Improve SCSS of Form Framework

Cleanup SCSS of Form Framework. Improve the usage of variables.
Furthermore, optimize the structure tree spacings of Form Editor.

Resolves: #82327
Releases: master
Change-Id: Ia1088475e16e5a04999140cca5ee3ff7d713e661
Reviewed-on: https://review.typo3.org/53962


Reviewed-by: Benjamin Kott <benjamin.kott@outlook.com>
Tested-by: Benjamin Kott <benjamin.kott@outlook.com>
Reviewed-by: Bjoern Jacob <bjoern.jacob@tritum.de>
Tested-by: Bjoern Jacob <bjoern.jacob@tritum.de>
Reviewed-by: Susanne Moog <susanne.moog@typo3.org>
Tested-by: Susanne Moog <susanne.moog@typo3.org>