Missing "backbone", how do I get it?

I was having issues. A developer looked at my site and wrote this:

“I see an error on your page referencing backbone not being installed. Do you have a plugin installed that requires backbone?”

http://toni-esl.com/

I have no idea if I have a plugin requiring “backbone”. I don’t know what backbone is. It doesn’t look like something the average WP user deals with. I went to the dev site and wiki.

What should I do?

(I am starting to have CSS rendering issues and error messages in the customizer for some themes. Don’t know if they are related or not. That is for another thread.)

Thank you