Fylo Dark theme HTML CSS Flexbox

Hello, according to the challenge I should have to develop this site with CSS grid but I have made it with Flex Box because I feel comfortable with it, I think this doesn't matter very much.

I have questions!

The background images named "bg-curvy-desktop.svg/bg-curvy-mobile.svg" are not applied because I didn't know how to deploy them and make them dynamic selection according to screen size. Would you please guide me on how to choose multiple backgrond-images by CSS?

Thanks for the awesome design! Love you Frontend Mentor!