While in the Customize screen, you may notice a section called JS Snippet & Custom CSS.

Here's the tl;dr (too long; didn't read) version of what to do with this section:

Is this part of your webpage literally filled with beesIt might be. There's only one way to find out. We can't stress enough that modifying the code in this section is extremely dangerous, and could seriously impact your site's functionality and appearance. It's safer to just leave this section completely alone. If something from this part of the Customize screen needs to be edited, we will gladly handle it for you.