The theme automatically used for new spaces, until overridden.

When a new space is created, Theme Press automatically applies a chosen theme to that space. (You can override this at any time within space settings.)

• Upon installation, Theme Press uses the built-in Lumen theme as the Theme Press Default Site Theme. 

If the Theme Press Default Site Theme is changed:

• all spaces using the default will change to the new theme.
• newly created spaces will also use the new default.
• spaces that have overridden the default site theme will not be changed.

• Changing the Default Site Theme