How do I enable the mobile version of the theme?

At this time, Radiant and Ballard are the only responsive themes. This means that for Radiant and Ballard, you can enable the mobile view to create an optimal viewing experience for your blog visitors.

For either theme, go to the "Advanced options" section of your Customization page.

Radiant:

Make sure you toggle OFF "Use default mobile theme" to enable the mobile view. 

Ballard: 

Toggle OFF "Use default mobile theme" to enable the mobile view.