Do you even have a fixed - non scrolling bottom tab bar working?  I've never been able to do that out of the box.  Huge limitation in the tab bar control to me.

Now I know Keith Strickland figured out how to fix a tab bar to the bottom.  I believe he's blogged it on his website but I've not tried to reproduce it as I'm now trying to use Twitter Bootstrap to replace Mobile Controls. 

But If you can get the bottom tab bar fixed in position I would guess to change the look and feel you'll need to do a bunch of Inspect Elements to find what css you need to override with your custom style. 

Just my 2 cents...