QR-Code with Flexbox

What are you most proud of, and what would you do differently next time?

Using css variables, which i never used before.

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

I'm struggling always to understand how flexbox works in detail. By researching with google i got sites to understand what i should do.

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

Is my css-code clean enough to understand what it does? Is there a opportunity to make it smarter?