The default theme is the one guests and members who have not specifically chosen a theme will see. Any of your themes can be the default theme.
To set a theme as default, navigate to Customization -> Themes, and click the Edit next to the theme you want to make default:
On the General tab of the Edit form, toggle the Default Theme? setting to On, and then save the form.
Note: Changing the default theme won't affect members who have already specifically chosen a theme. To change that, follow the instructions below.