Theme Removal
1. Login to your site’s CPanel > File Manager > public_html/wp-content/themes
2. Find the erroneous theme’s name. Click on the theme then click Compress – if you look at the top menu one of the options is compress – you need to compress your file for backup purposes.
3. Click your theme’s name again then click Delete, delete is just inline compress button.
By deleting the current active theme, WordPress will revert the theme to another theme automatically because it cannot find the active theme.