[BUGFIX] Apply proper button class to BE Layout Modal

The modal “OK” button of the BE layout column assignment now sports the correct color
for the button.

Resolves: #83662
Releases: master
Change-Id: I13587241f5f79a555ef7edab6071d1d3f0265ad0
Reviewed-on: https://review.typo3.org/55532


Tested-by: TYPO3com <no-reply@typo3.com>
Reviewed-by: Jan Delius <me@jandelius.eu>
Reviewed-by: Stephan Großberndt <stephan@grossberndt.de>
Reviewed-by: Henrik Elsner <helsner@dfau.de>
Tested-by: Henrik Elsner <helsner@dfau.de>
Reviewed-by: Reiner Teubner <rteubner@me.com>
Tested-by: Reiner Teubner <rteubner@me.com>
Reviewed-by: Michael Oehlhof <typo3@oehlhof.de>
Tested-by: Michael Oehlhof <typo3@oehlhof.de>
Reviewed-by: Mathias Brodala <mbrodala@pagemachine.de>
Reviewed-by: Rico Drexler <rico.drexler@gmx.de>
Reviewed-by: Andreas Fernandez <a.fernandez@scripting-base.de>
Tested-by: Andreas Fernandez <a.fernandez@scripting-base.de>