QR code challenge solution using basic HTML and CSS (First-time)

My main questions concern the different colors background and The image:

• How can I get a white backgroud on top of the body background directly in CSS i
• What is the best way to style an image so it fits the designated container for it? (Like did I need to have the img inside a div in my code so it can fit in the middle)
• How to approach this problem with both the web version and mobile version in mind?