Removing borders from images in Matala theme

I would like to remove the borders from all photos in the Malta theme. I do not have the custom upgrade so I would have to make changes within my CSS stylesheet. I have read every post I could find on this topic and have tried everything suggested but am not achieving the result I want. I am new to this process so I hope that I am using the correct terminology. My website is blog.livbydesigninteriors.com

Thanks!