Regretfully there is no option to change the Topbar background color but you can use custom CSS code to change it. You can use this custom CSS code under Theme option —>CSS.
border-bottom: 1px solid #37898c;
Please feel free to change the color code as per your design.
The purpose of providing custom CSS is to show you how to add CSS code to your site. Writing custom CSS is outside the scope of our theme support. If you need more customization, you need to learn CSS and learn how to use the browser’s element inspector or you can subscribe to One to get further assistance.
Hope it helps.