You can switch themes in
Configuration → Theme & Navigation, where you'll find three settings:
- Site Theme
- Mobile Theme
- Fallback Theme
Site Theme is loaded on desktop and tablets. The
Mobile Theme is loaded on mobile devices, while the
Fallback Theme is used when creating a new custom theme.
Any files not present in the selected site and mobile theme will be loaded from the fallback theme. Unless you are creating a new theme from scratch, the fallback theme should be the default one.
There isn't a dedicated mobile theme because the site theme is responsive and works well on all device sizes. There used to be a mobile theme in the past and for that reason we continue to keep the setting there because it can also be useful if you really want to have a different output on mobile, rather than just have the layout be responsive.
Follow along and get started with theme customizations in just a few steps.
Here's a list of useful theme and template customization resources