Solution from stvwlf - Cheers
current_theme_info is found in file /wp-admin/includes/theme.php
The errors suggest that file is missing.
If you don't have it in your WP installation you can start by extracting it from the WordPress files zip file and replacing it, and seeing if that resolves your error.
If one file is missing though, others are likely to also be missing. If you get any additional errors I suggest uploading the WP files again, overwriting the existing ones. Do a fresh download of the files rather than use a copy you may already have on your computer. If you are using the default or classic themes back up your wp-content folder first.