You can actually enable debugging from your config.php file in your moodle install folder. I am presuming that you have access to the server. See here:https://docs.moodle.org/30/en/Debugging
You might also try entering a url to a specific page and see if that loads. I would suggest maybe deleting whatever themes you have added one by one - found in moodle/theme folder on your server. Do not delete Clean or More as those are the built in ones.