TotalCMS. CSS for Image modal background and gutters in Cards layout

I have just built an artist’s website using Uikit 3 and TotalCMS. I am pretty useless when it comes to making minor changes with CSS and hope I someone here will offer a solution to a couple of minor problems.

I have created several galleries using the TotalCMS feed stack set to display the ‘Cards’ masonry layout of thumbnail images and text. When a thumbnail image is clicked a modal window pops up to display the full size image. Currently there is transparent background. I want this back ground to be a solid white. Might there be some CSS to adjust this?

Also, I would like there to be larger gutters between the seperate ‘Card’ items in the thumbnail gallery? Perhaps there is some CSS to adjust this as well. Any advice would be greatly appreciated.

The test site is here: