Funny, I have almost the exact same question. Seems like it should be such a difficult thing and shouldn't require defining a CSS class, but that's about all I've been able to find in the form of an answer.
I dont know if my code Will help you.. But I can tell you what I did exactly..
so firt install the firebug plugin for firefox.. then when you run your project, with firebug you can see whats the id or the name of your contentpanel..you just move your mouse over your window and the panels become blue, and in the window under it there is the id or name..
Then when you found that..for example in my project it was: "x-tab-panel-body"
in your code, you give your panel another name... so : pnl.setStyleName("myPanel")..
and then I modified the CSS-data, i just input this: