Website broken - URGENT

I have a current website running another theme. I downloaded illdy.2.1.1 activated it and the website is now unavailable. I cannot access the dashboard to de-activate the theme. My web host company has picked up the following error message:
14931 [Wed Jul 04 [error] [client] AH01215: PHP Fatal error: Call to undefined function add_editor_style() in /home/sites/ on line 97: /home/sites/
which they think might be related to the broken website.
Please can you advise as I have no way to get into the admin to deactivate the theme. Obviously, this is urgent for me. Can you also advise why the theme wasn’t working and if it can be corrected.

hi Sally

Im not sure how or why this happened but you can delete theme folder in wp-content/themes folder by ftp browser or from your hosting panel file browser