GPII-523: Updated contrast theme adjuster.

Specify model values as defined in https://docs.google.com/spreadsheet/ccc?key=0AppduB_JZh5EdGltZnF3dVpKdXcxSVhEZ0VjZGY1U3c&usp=drive_web#gid=0, line 23.
Do not allow inversion theme as it is not supported in common terms currently.
Leave a default theme selectable until http://issues.gpii.net/browse/GPII-566 is resolved.