What files would I need to edit to add a div element to the HTML files? I’m using the theme as just a basic one page – no different sections – and I need a full screen background image that scales responsively, with live text that also scales down responsively. The only way I think I can do this is by editing the structure of the page. Can you advise?
I would sincerely recommend you to set up the default homepage first by selecting a blank page in Dashboard > Settigns > Reading as the Front Page Display.
Then you should hide all the sections but one in Dashboard > Appearances > Customize and then add your text in one of the sections.
If the default section widgets does not work for you, you can use a text widget for any HTML syntax.
This way you get the homepage jumbotron responsive header image and the content below it.
If you want to do some custom coding, have a look in the theme folder on the server > front-page.php
There you can modify the homepage to your needs.
I hope this is what you were looking for and gets you forward with the setup.
PS: You might want to consider a child theme if you are working on the theme files so you can proceed to update the files in the future