1. If you have made any modifications to the theme custom.css, temporarily remove them.
2. If that does not solve the issue, try deactivating all plugins.
3. Finally, if issues persist. you may need to re-download the theme and upload a fresh copy. Always deactivate the theme before attempting to re-install it. If you have access to your web space via FTP, it is best to upload the theme folder so you can delete the old files first and be sure the new one uploads completely and without error.