The default grayish borders are just a little too stark for me.
You really should be thinking of what's good for visitors to your site, rather than your own tastes ... and the preferred look for e-commerce sites these days are a clean uncluttered look.
The background image thing is a bit old fashioned. It looks pretty amateurish and tacky to me.
But if you insist ... this is the CSS that site is using (In Chrome, right click and click "Inspect")
@media (min-width: 768px)
body {
background: url(//8smallpaws.com/wp-content/uploads/2018/05/background.jpg);
background-attachment: fixed;
background-repeat: no-repeat;
background-size: inherit;
background-position: top center;
}
Just upload your own background image and edit that code above. You will just have to experiment with the image size. At least with the "@media (min-width: 768px)" it won't show in mobile ... which would be even more cluttered.