You can visit https://anchor.hecker40.repl.co on mobile to see that part of the background image is white. I've been trying everything to make it full height, but it just isn't working.
@media (max-width: 600px) { section.main { background: url("https://anchor.hecker40.repl.co/bg.jpg"); }
I tried height:100%; and almost every background property and it didn't work.
I'm not entirely sure what you're trying to do here. From what I can tell there is a section in the middle of the page where the background doesn't cover the whole section.
To fix this, add background-size: cover;. This will center the background in the element and have it scale all the way to both ends (while keeping proportions).