In combination with the module 'Mobile Device Detection " this module can change themes according to the device (phone, tablet, desktop).
Convenient... because not all themes are mobile friendly and on the other hand, not all mobile themes are handsome on a desktop.
First you create a 'rule':
Give it a name and choose a (active) theme. You can also set additional conditions for pages here. A * means that you want to appeal to all pages.
Now can you determine other conditions. These are from the Mobile Device Detection module. I choose for special devices. You can switch by language, device, content type and user role ... If you can program you can make other conditions. I refer to the explanation on the page module.
Here I decide that I want to use Bartik on mobile devices. The other devices will use the default theme (here Adaptive Theme)
The condition looks like this.