This may help someone...
I had the same issue when I tried to switch from paper_lantern to jupiter theme, the blank screen appeared and I couldn't do anything further.
Reaching my hosting providers wasn't an option at that moment as I needed to fix something very urgently.
As a workaround, I noticed that, when you try to switch theme, cPanel actually makes a POST request to /execute/Themes/update while passing the new theme in the body of the request.
You may imitate this request using any HTTP client like Postman.
You also need to copy the cookies from your browser and add them to the Cookie header of your HTTP request so that the request is authenticated.
I was able to do this and revert back to paper_lantern without reaching out to my hosting providers.
Hope this helps someone.