Increase size of galleries

SITE: https://www.james-kendall.co.uk

Hi, I wonder if anyone can help a CSS novice. I'd like to increase the size of the pictures in my gallery slideshows like this one so they take up the full browser width without cropping. I've tried some code posted to this forum, but I'm doing something wrong because it isn't working for me. 

Mobile is fine (though the gaps between the images are a bit big - not the end of the world).

Thanks for any help.

James

Feel like I should be able to do this, but can't  get there. If anyone has any guides I'd happily give it a go.

Try adding to Website > Website Tools > Custom CSS

@media screen and (min-width:901px) {
#fullscreenBrowser .slideshow .slide img {
    width: 40% !important;
    height: auto !important;
    top: 50% !important;
    left: 50% !important;
    transform: translate(-50%,-50%) !important;
}
}