[BUGFIX] Installtool: add bottom margin for content-area

Add global bottom margin for the main content area.

Releases: master
Resolves: #71384
Change-Id: I5caf82dfe3fcf228fea7c97df23102bbe7652c8c
Reviewed-on: https://review.typo3.org/44723


Reviewed-by: Harry Glatz <glatz@analog.de>
Tested-by: Harry Glatz <glatz@analog.de>
Reviewed-by: Oliver Eglseder <oliver.eglseder@in2code.de>
Tested-by: Oliver Eglseder <oliver.eglseder@in2code.de>
Reviewed-by: Daniel Lorenz <forge@extco.de>
Tested-by: Daniel Lorenz <forge@extco.de>
Reviewed-by: Georg Ringer <georg.ringer@gmail.com>
Tested-by: Georg Ringer <georg.ringer@gmail.com>