Change border color around Classified listing pictures

I have my classified website setup to display 3 columns of listings. How can I change the border around the photo to black instead of white? I’ve looked in the style.css file and I cannot locate it there or in the ‘Color Changes’ option. I’ve also tried to add code to override it but it doesn’t work. Seems like this should be simple but I can’t find a solution. Here is my page:
https://dragboatcity.com/new/listing-category/boats-for-sale/