Is there a way to add new div class via a theme’s Additional CSS interface?

I am trying to modify a woocommerce website and I have very little HTML/CSS training. Most of what I’ve learned has come from trial and error, some online tutorials, playing around with the developer console in Chrome, etc….

I’m trying to add an image border to the top of a page header right before an area called tg-container. I was able to put one below it, using an existing class, whose info I didn’t need to see. Can it be done with the Additional CSS interface built into the theme or do I have to go into the style.css and html to create the boarder? I know the benefit of the Additional CSS interface is having no need to edit the theme files directly whenever the theme is updated.

Bypass Page

Example Page with border – accessible only after bypass page is visited

IMAGE: Example page with Chrome Developer Console open and showing code for area