How do I add a black line to the bottom of a banner image

I would like to add a thin black line or a thin space between the banner images on the index page. I have tried adding spacers, but that doesn't work. I cant add the line to the image because of how the images crop on different devices. Is there a simple way to do this?The site is https://coral-magnolia-rgj9.squarespace.com and the password to view is FBHSsite. The template is Fulton

Any help is greatly appreciated

@FindersBeepers Add to Home > Design > Custom CSS

.homepage .index-section.page {
   border-bottom: 10px solid #ff0000;
}

Perfect. thank you so much for your help