Can't get header to resize on mobile

Yes, I need help again ??

I’ve read other topics but can’t get the solutions to work. My header image is parallax and on mobile the image is way too big. I can’t get it to shrink down even when I try using CSS. Maybe I’m targeting the wrong class? This is the type of code I’ve tried:

.custom-header {
  background-size: 100% 100%;
}

@media only screen and (max-width: 520px)
.custom-header {
    min-height: 92px;
}

That was taken from another post but it made no difference on my own site. I’d appreciate any help.