Switching theme on mobile devices

I have a site based on Elgg - which is using the River Addon theme to give 3 columns on the main site and uses Mobilze to create the mobile site.

My problem is that Mobilize goes not give the navigation tree for pages (which is the main content for the site). The Aalborg theme does this nicely on mobile devices but isn't what I want for the PC based site.

My question, then, is can I switch the theme for a mobile device - maybe using the IsMobile function....

Allborg on Mobile and RiverAddon elsewhere would be perfect - any suggestions welcomed