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

Ok, I got it figured out. I added <div class="title-border-top"> </div> above the page title (and actually added <div class="title-border-bottom"> </div> below for future modification.

I then added the following in the Additional CSS:

.page-header .title-border-top{
    background-image: url("https://store.dolina.org/wp-content/uploads/2020/08/wall-murals-floral-polish-folk-art-pattern-in-square-wycinanki.jpg-e1596302126833.jpg");
    background-repeat: repeat-x;
    height: 30px;
            margin: 10px;
}

It is working and I hide the entry-sub-title I had taken over for the border to begin with. Now I have to figure out which page templates WP is pulling for all the main pages, so I can add the html to them.