I have tried to import the demo content by deleting the previous site contents but the demo content of the NEW theme is not either importing everything or it’s not loading correctly.
can you reset the site, make sure you have no Cache Plugins or Server Side caches enabled and the re-import the site. Check the site before re-enabling any caches.
Yes, make sure any caches ( plugin and server side ) are disabled.
Then reset the site.
Then import the site.
Check the site. If its all good. Then re-enable your caches.