How do I change themes in Anchor CMS?

Let's say I have two themes in the themes folder, default

and custom

. I know that I can switch my current theme to custom

by renaming the folder to default

and the original folder default

to something other than default

.

But is there a way to configure it somewhere in the config file or in the admin menu? I tried looking in the documentation which mostly talks about creating themes but doesn't switch.