Chat-app illustration Html & Css

after spending a couple of days trying to style the body with background property. I finally figured I was styling it in the wrong way I think, so I created a div with an empty element for the background to make it work, I don't know if this is the best practice, I used HTML & CSS for the project, would have loved to use sass but I am still a beginner.

any thought on how I can improve better on my code. pls would love to hear it. thank you