[BUGFIX] Install tool shows docheader in backend

Install tool should show a document header if called in backend.
It includes some .css files from t3skin to do that. Those files
were moved around in issue #51134, not adapted in install tool,
resulting in 404 and missing document header. The patch adapts
templates to include files at new locations.

Change-Id: I9df6950ec3e218c175b012d9ef4233c5b5f6c925
Resolves: #51952
Related: #51134
Releases: 6.2
Reviewed-on: https://review.typo3.org/23789
Reviewed-by: Christian Kuhn
Tested-by: Christian Kuhn