[theme expound] trying to add border round content

Hi there

I have installed a new Expound theme on my website https://wwww.thetechhouse.co.uk. I have added a banner and it has a 1pxl black border around it. I was wondering if there would be any way to add a border around the white square my main content is in, in order to seperate my main content and background image.

Any help would be appreached thanks!