I've been interested in this too. I haven't had a chance to take a look but I think that the place to start would be the sass files used to generate the css, specifically...
...found in resources/themes/stylesheets/sencha-touch/default/core/_core.scss
Note, this is completely beyond the use case, is unsupported, and I haven't even tried it, so you mileage may vary on this rough terrain should you choose to pursue it. But let me know how it goes ;-)
Also, I could be completely wrong.