The problem seems in how js and css are aggregated in a single file
(
http://blog.orbeon.com/2007/03/combining-javascript-and-css-files-for_11.html)
Following the post if you put the line
<property as="xs:boolean" name="oxf.xforms.combine-resources" value="false"/>
in properties-local.xml
the feature is disabled and you get back your css files (with media attribute).
Cheers,
Alberto.