fix bug 242576: cannot unset background colors
fix bug 242576: cannot unset background colors

Anders already fixed this issue already in bug #97400 in January 2005.
In October 2005 Hamish overwrote this again accidently. Hence, this
bug was back for 6 years now. Finally, fix it again.

BUG: 242576

sandsmark/kate,hlamer/kate,DickJ/kate,sandsmark/kate,hlamer/kate,hlamer/kate,hlamer/kate,jfmcarreira/kate,hlamer/kate,jfmcarreira/kate,DickJ/kate,DickJ/kate,jfmcarreira/kate,hlamer/kate,DickJ/kate,cmacq2/kate,cmacq2/kate,hlamer/kate,sandsmark/kate,hlamer/kate,sandsmark/kate,hlamer/kate,cmacq2/kate,hlamer/kate