In the global theme configuration screen, the "Default Theme" means the standard state-of-the-art look and feel that Confluence ships with. I'll refer to that as the "Standard Theme" in this description.
In the space theme configuration screen, the "Default Theme" is correctly described as "the globally configured look and feel." Unfortunately, if the global theme is not configured to the Standard Theme, then a space cannot be configured to use that theme.

To replicate:

1) Set the global theme to 'Confluence Classic Theme'
2) Set the space theme to 'Confluence Classic Theme'. This configuration works.
3) Switch the space theme to 'Default Theme'. The space appears similar to the Classic Theme.