Responsive QR code website using CSS Flexbox

What challenges did you encounter, and how did you overcome them?

Aligning the container vertically and horizontally was a challenge. I aligned it horizontally using "justify-content: center" in CSS styles, but vertically it was not aligning to the center using "align-items: center" was not working. So i used "margin-top" to align the container vertically to the center of the page

What specific areas of your project would you like help with?

I would like to get help with alignment issues I have, aligning the container to the center always troubles me. I don't think i am using flexbox correctly. I don't understand under which selector I should add display type as flex and use justify-content and align-items.