Centering the whole thing. But with a flexbox it was pretty easy
#body{
height: 95vh;
display: flex;
justify-content: center;
align-items: center;
}
I'm not a master in front end, just started my journey as well.
However, I have some feedback. Let me use it as an exercise for me when doing code review.
I see that you give id named #body on a div element, this gives a redundant feeling with the body element of the html.
Also HTML semantic is not very well specified because there are three levels of div (not sure what's the significant though, but I heard it's a good practice to use a good semantic for the doc).
There is blank lines on the css #qr-container.
Also, I think using id to give style to the element is not recommended. id should be used to give unique identifier on the element rather than specifying style, class is recommended.
