Mac OS Mojave Terminal Resetting to Default Theme

I’ve recently worked out how to change my terminal theme and set the new theme as default through the GUI menu: Terminal > Preferences > Profiles > Default. This works and sets the current session to that theme but after restarting Terminal the theme resets back to basic.

I’ve tried deleting all other themes and setting a specific one, and although this will work for the remainder of the session it does not work after restarting the app.

I can’t seem to find any specific help for this issue. Others have asked about setting a default or it not working on multiple windows/tabs, but that isn’t the issue here.

Is there a config file where I can change the theme manually?

Any suggestions will be very much welcomed! Working on a 2017 MacBook Pro 15″ (MacOS Mojave)