[BUGFIX] Follow-up: "edit this column" again

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

Fixes regression in row-header icon color from
I4d189857c2fd884e19479a5d702671bdc0e2b3da

Resolves: #63472
Related: #63468
Releases: master
Change-Id: Iaeac45e3594fd4f3fa6c17c80146b4d070600c5e
Reviewed-on: http://review.typo3.org/34840
Reviewed-by: Georg Ringer <georg.ringer@gmail.com>
Tested-by: Georg Ringer <georg.ringer@gmail.com>
Reviewed-by: Benjamin Kott <info@bk2k.info>
Tested-by: Benjamin Kott <info@bk2k.info>