[BUGFIX] Show icon "edit this column" again

Change the color of the icon to dark instead of white to make it
visible again.

Resolves: #63468
Releases: master
Change-Id: I4d189857c2fd884e19479a5d702671bdc0e2b3da
Reviewed-on: http://review.typo3.org/34831
Reviewed-by: Felix Kopp <felix-source@phorax.com>
Tested-by: Felix Kopp <felix-source@phorax.com>