You would have to do it programmatically, AFAIK.
Something along the lines of:
$(document).bind( "mobileinit", function ()
{
...
$.mobile.page.prototype.options.contentTheme = "z"; //your theme
...
});
Now, since there is no centralized hook - you will have to do the similar line for all theme options there are:
$.mobile.page.prototype.options.headerTheme
$.mobile.page.prototype.options.footerTheme
and so on.
I don't have a list of all of them, but a quick look through the jquery.mobile-1.0rc1.js searching for .prototype.options.
reveals these:
$.mobile.page.prototype.options.backBtnTheme
$.mobile.page.prototype.options.headerTheme
$.mobile.page.prototype.options.footerTheme
$.mobile.page.prototype.options.contentTheme
$.mobile.listview.prototype.options.filterTheme
so it seems to me that you can go with these and discover more as you go. Note that not all of them are created like that - some are constructed dynamically in the code. Look for Theme
string to see what I mean.
Update
$.mobile.page.prototype.options.theme
should be updated as well - based on Moak's comment below.