Flex Baux, go! HTML & CSS

Is there some dark magic secret to getting the design to match the previews better? First solution looked amazing on my screen then the upload to F.E.M. looked... less...amazing.

Flex box was easier to implement/play with than I expected. Still a few edge kinks I need to play with. Is there a way to prevent individual boxes from growing? Is there a better "best-practice" way to control padding, height, etc?

Desktop used VH for overall height but that broke mobile version so I switched to 100% and it seemed to work well for me but maybe there's a better way to control this?

Tear it all down and let me know anything I did wrong/ that could be improved! Thanks!