[BUGFIX] Install tool - Open Sesame door - styling

Dismantle custom styling for backend module lock screen.
Base on core wide standard styles.

Resolves: #63251
Releases: master
Change-Id: Idbc65c896732195fc854763c4dd8b37f6e3e6d4e
Reviewed-on: http://review.typo3.org/34798
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>