Lulu here from the Shopify Support team. Thanks for reaching out here, and I'll be more than happy to help you out.
Thank you for the screenshot, that does seem like quite a strange issue. Can you clear your cache and cookies and ensure you are using Google Chrome and try to add a theme again.
Once you've updated your browser, would you mind sending a video of how the admin is behaving on your end when you add a theme again, please? You can use a third party application something like Screencast-O-Matic to do this. Once I can see the issue on the video, we can look at possible causes and help you resolve it.
Lulu | Social Care @ Shopify
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If you cant get free theme from store then you can get it from here also.
Let me know if you cant.