- Download a copy of the theme - if it's available in the WordPress repository, grab that. If it's a premium theme, have the client provide their credentials to download, or just have them provide the .zip file directly. Make sure that you obtain the exact version that is being used on the client's site - not the latest version.
Then, download the theme they're currently using, and use a file comparison tool such as BeyondCompare to compare the files. Make a list of whichever ones are different.
- The only safe way to proceed is to create a child theme. You'll want to copy only the customized files into a separate folder, and create a very simple
style.css file with comments at the top that tell WP this is a child theme of whatever the parent is.
Then, you can delete the parent theme, upload the "clean" copy (the one you obtained directly, not the one they were previously using), upload the child theme, and activate the child theme.
From then on, you can safely update the parent theme as often as needed. Occasionally you may still need to update the child theme - whenever the parent makes significant changes to the files that are in the child theme.