When you change Confluence's global theme from its own Default Theme, spaces no longer have access to that theme. In other words, individual spaces cannot use the Confluence Default Theme. (See CONF-10208 for details.)
With the Brikit Default Theme add-on, you can apply the default look and feel of Confluence to a space. Installing the add-on is easy. After installation, review Atlassian's page on Applying a Theme to a Space.