GPII-1015: Invert header colours

Inverted the contrast of the panel headers for the adjuster groups. 

This pull request does not address the inconsistent implementations of the panel headers. It also introduces a hack to ensure that the panel headers heights were the same (for some reason in the PCP they were 6px shorter despite the markup and styling being essentially identical).

http://issues.gpii.net/browse/GPII-1015
The new class is no longer needed with the removal of the hack for missing 6px.