noobie-made QR code challenge

Hey, everyone, I've been learning HTML and CSS for a week now, but have decided to make some practice to understand the whole thing better. Can't even say what was the hardest part to make to qr-code itself or to figure out the GitHub. Was really close to burst into tears at some point.

Anw. I was pretty much confused with the mobile version, I remember being taught of media queries, however I'm not sure I used it right. Also the panel size, is it okay for it to be exactly pixeled? Or should I advantage percentages? Just saw a p in CSS I do not need at all.

So, in conclusion any feedback at all would be appreciated. :)