Flat Iron: Replace the Header Background with an Image

Hello, I'm previewing the Flat Iron template on a version 7 website and wanted to see if there was a way to use an image in place of a solid color for my header's background.

I found some custom CSS code for Brine templates but can't figure out how to alter it to get it to work for Flat Iron. Any ideas? Thanks!

.Header {background-image:url('your-URL-here'); background-repeat:no-repeat; background-size:cover;background-position: center;}

Add to Design > Custom CSS

header#topBar {
    background-image: url(https://cdn.pixabay.com/photo/2021/07/09/06/57/lavender-6398425__480.jpg);
    background-size: cover;
    background-position: center center;
}