Here's a code that did it for me:
<style name="AppTheme" parent="AppBaseTheme">
...
<item name="android:typeface">serif</item>
...
</style>
Look here for what fonts are available by default on android. From what I read adding other fonts is not possible without sub-classing your views, so I can't help you there.
EDIT: I may be wrong. Check this out: http://www.jayway.com/2013/01/22/custom-typeface-in-android/ and here: http://www.androidguys.com/2008/08/18/fun-with-fonts/