I was playing with ASP.NET MVC Mobile to start something quickly for my phone.
I then updated the NuGet packages, and my style was GONE.
Seems that jQuery 1.9.* is incompatible with jQuery.Mobile 1.2.
According to the website 1.3 is out, so the package should soon be there, so until then you can revert from jQuery 1.9.* to 1.8.3 with the following code in your Package Manager Console:
uninstall-package jQuery -Force # ignore that some have dependencies on jQuery install-package jQuery -Version 1.8.3
Have a good one,