Switch to other theme at URL

I want to switch to another theme at when a user visits www.mysite.com/m/. Then, redirect mobile users to www.mysite.com/m/ with htaccess based on their user-agent.

I have the htaccess redirect rules and it is working for me. But how do I switch to mobile theme based on the URL?

The Virtual theme plugin is a great solution for me, but unfortunately it doesn’t work for mobile browsers.

Given that this is an older question and the mention plugin is very much depreciated, both the WordPress core and themes have changed greatly since then.

Nowadays, most themes are mobile-friendly and will conform based on the amount of screen space is being used to visit a website.