Switch Themes Using a Static Url

I don’t want to use heavy plugins. I’m customizing my own theme and css for a mobile version. I’d like for the mobile version to auto detect users on mobile devices, but also at the footer have a link “Full Site Version.” I’d like for the user to be able to switch to normal site view theme. I’m not sure how to code this part. I’ve seen it done on some plugin themes, but given that I’m making my own design… how do I make a static URL to switch to standard theme version.