How to Customize Gallery Lightbox

I am using the free Fotograpfie theme on Wordpress and some of my gallery images become blurry when pulled up in the lightbox. I thought maybe the ones that were more blurry were either really small files or really big ones, but it is a mix of sizes and still nothing extreme at all. I also want to add some padding around the image in lightbox because right now it pressed right up to the full height of the page. Thank you very much for any help.