Fix themes for gradient. For other (contrib) theme maintainers: back property is not used for 3D viewport back colour anymore, instead use the gradient high colour, as done in this commit

git-svn-id: https://svn.blender.org/svnroot/bf-blender/trunk/blender@54439 954f8c5b-7b00-dc11-b283-0030488c597c