Preview Background styling

* Added background color to preview for sites that don't have a
background color (based on @clarkepaul's commit:
https://github.com/silverstripe-droptables/sapphire/commit/7750c8f75fc4a
764b384c6e19dbe543c4811cdcb)
Merge pull request #2060 from adrexia/preview

Preview Background styling