How To Make Images In A Gallery Lightbox Bigger With CSS

I am trying to make the images in my gallery masonry page appear bigger in the lightbox, when you click on the images in the lightbox they look smaller in than they appear on the page itself. :

i'm trying to make them fill the length of the screen or just bigger in general, they're pretty big images (ex: 18in x 24 in/12 in x 16 in), the smaller images (5 in by 7 in) appear way bigger in lightbox than the larger ones....(and yes I have reduced the sizes of the bigger images to smaller sizes but it still appears the same way)

https://www.erinncapko.com/

(the main page/illustration. page is one the trying to fix, thank you!)

10 hours ago, Ziggy said:

You can try this Custom CSS:

.gallery-lightbox {
  padding:0px;
}

Thank you so much! Yes, this worked perfectly.