How to add a sub-theme for mobile version?
Create another subtheme (lets call it mobiletheme ) and with mobile theme module set your mobile version of site use mobiletheme.


  Adds the ability to choose a mobile theme in
  admin/build/themes/settings which is used for serving users on mobile
  devices.


another solution is detect and set theme for mobile with themekey module (Check  tutorial here: http://mustardseedmedia.com/podcast/episode52 )


  ThemeKey allows you to define simple or sophisticated theme-switching
  rules which allow automatic selection of a theme depending on current
  path, taxonomy terms, language, node-type, and many, many other
  properties. It can also be easily extended to support additional
  properties exposed by other modules. In combination with Drupal's
  theme inheritance you can easily achieve features like:

individually-styled channels
a front-page / "splash" screen
a date/time-selected Christmas theme
mobile themes for different auto-detected mobile devices
special themes for "limited" or "old" browsers
content, user, or role -specific themes
indicating your environment (production, staging, testing, sandbox, … )
testing your redesign safely on a live server
...

  
  And unlike other theme switching modules, ThemeKey should play well
  with internal and external page caches, like Boost or Varnish, even
  for anonymous users.


but really it is heavy module and if you want only want switch to another theme when user surf site with mobile I strongly recommend you to use   mobile theme module.