I try to use the app search tool plus looking into settings.
First, I’d suggest going to settings and checking SEO/Metatags tab and scroll down to the Advance section to check the script in header and body sections.
If you don’t see anything there, can search the app search tool for HTML elements, but I say that only because that is where I place my custom CSS so I can easily find it using the app search tool. Some developers may use the page settings and add it there, so might need to check each page.
If you don’t find any HTML elements with the custom CSS, and no pages have the settings, then I’d look at the plugins in use, delete any for CSS, then check the issue checker to see where the plugins might have had their values used by dynamic expressions.
Or, you could just contact the template developer and ask them where it is. Check the template page and go to the contributor profile and find their contact button/info.