Problem with closed caption button

We've created a custom skin for a player which uses the captions button.
Unfortunately we don't seem to be able to change the appearance of the cc button that appears in the player bar. Strangely enough we can style it when it is not in the dock. Here are two examples to show what I mean:

http://test.dev.bureau-va.net/jwplayer_dock_true/dock_true.html

http://test.dev.bureau-va.net/jwplayer_dock_false/dock_false.html

How can we style the cc button in the dock?