[BUGFIX] Docheader delete icon in EXT:scheduler is white

The delete icon in EXT:scheduler while editing/creating
a task is white instead of black.

Resolves: #63001
Releases: master
Change-Id: I5566db3268d8d3b2230c88f753cfc2410595c619
Reviewed-on: http://review.typo3.org/34235
Reviewed-by: Christian Kuhn <lolli@schwarzbu.ch>
Tested-by: Christian Kuhn <lolli@schwarzbu.ch>