Editing the margin for the gallery

Hi. I am doing a version of a website and we are open to purchasing the pro version BUT we just have to make sure we can get it to what we want. So first I struggled with adding text on top of the images but then I found that the sidewinder theme basically is perfect for us (https://www.imagely.com/wordpress-photography-themes/sidewinder/) but the small issue with that is the fact that we want to have say a 20px border so more something like this https://www.imagely.com/wordpress-gallery-plugin/basic-thumbnail-gallery/ but double thickness. Is this something we could do easily?