Blank multisite admin caused by missing default theme?

I’ve followed the setup and installation for multisite but the network admin dashboard, users & site appear blank.

I’ve checked the wp-config.php & .htaccess and all seems fine.

I understand from previous support forum threads that it might be caused by having uninstalled the default theme.

However it seems the only way to reinstall a theme is through the network admin dashboard which now I can’t see.

Is there any way to install a default theme without having to completely reinstall wordpress?

Any and all suggestions would be most welcome.

Many thanks