When you try to enter the editor for this imported theme, can you try this?
Edit your theme - if theme loader is stuck loading (and the UI is frozen), open your browser console. If in Chrome, you can click and hold down the refresh button, to see a few refresh options. Choose to “Empty Cache and Hard Reload.” This has helped with similar issues in which the theme loader won’t load completely.
You had mentioned that the theme looks different in your sandbox than it did before - Would you mind looking at the theme file you imported into your Prod org, and check the file size? It should be a few hundred kilobytes, I believe. I’m wondering if you’d be able to re-export the source theme, now that you have the permissions settings necessary for theme management, to see if the exported file looks different.
