QR Code page

It doesn't look bad at all, but there are some quick changes I would implement:

1. Use the included "style-guide.md" file. It gives you the correct colors, font family/size, etc. Colors can also be extracted using a color picker.
2. While your solution of adding the image as the background image of the "qrcode-box" div works, it would be clearer and more accessible if you added it directly to the HTML file, using the "<img>" tag.
3. I think it would simplify your life if you, using the universal selector, *, set "box-sizing: border-box".
4. If you set the height of the "container" div to fill the entirety of the viewport height using "height: 100vh", the card (or "general-box" in your code) would also be centered vertically. Always.
5. The bottom corners of the card doesn't have rounded corners. ;)

I really do not think there's any need to add a flex-direction property to container seeing as there is one "general-box" in it. Also I'd also suggest you rely on the intrinsic sizing of the general box. Also the media query seem redundant as it is going to be displayed in the center because of the flex properties you defined.