D20766: Use appropriate background color for text previews

[Kai Uwe Broulik]

broulik added a reviewer: kossebau.

REPOSITORY
  R320 KIO Extras

REVISION DETAIL
  https://phabricator.kde.org/D20766

To: eshalygin, kossebau
Cc: broulik, kde-frameworks-devel, kfm-devel, alexde, feverfew, meven, 
michaelh, spoorun, navarromorales, firef, ngraham, andrebarros, bruns, 
emmanuelp, mikesomov