[BUGFIX] Hover ExtJS buttons bg-color inherit

Fixes ExtJS buttons that had dark bg-color on hover due
to Bootstrap+FormEngine refactoring.

Resolves: #63226
Releases: master
Change-Id: I331477dadc5948b52108977518aa3a214009ad1a
Reviewed-on: http://review.typo3.org/34794
Reviewed-by: Benjamin Mack <benni@typo3.org>
Tested-by: Benjamin Mack <benni@typo3.org>
Reviewed-by: Mathias Schreiber <mathias.schreiber@wmdb.de>
Tested-by: Mathias Schreiber <mathias.schreiber@wmdb.de>