I just recently installed Discourse. I’ve been looking through the menus. I know how to EDIT a Discourse theme (Admin page -> Customize), but not how to actually set it in the Admin page (I want to change from Dark to the default white theme).
From the admin panel, there’s a checkbox in the theme that reads
Theme is enabled by default. Check that and save the theme.
Discourse also has a selectable user default theme setting as well in the user settings under
user preferences->interface->theme, which allows a user to select their own default.