[BUGFIX] Install tool: Fixed document header

The document header is not at a fixed position but "scrolls away".
Fix this by adding a missing t3skin structure .css that defines this.

Change-Id: Ia1b3209db1b5fa08157002bca950110830e2403e
Resolves: #51989
Related: #50910
Releases: 6.2
Reviewed-on: https://review.typo3.org/23812
Reviewed-by: Ernesto Baschny
Tested-by: Ernesto Baschny
Reviewed-by: Wouter Wolters
Tested-by: Wouter Wolters