Thank you guys. Other than giving Firefox permission to access the root folder, it turns out there was more to do in order to get OperaGX theme to work, like enabling a few more config settings, and copying the entire chrome folder and user.js file from the source.
If you use Windows, you can just do the following:
And you’re done! You do not need any special programs or anything like that because Windows already has a built-in one.