Blind I don't have this theme to test, but assuming the CSS selector is correct, maybe you have an issue where two margins are superimposed on each other.
You could try to put an even larger value to see whether anything starts happening. Maybe it overflows with the 50px padding already present to offset the header.
Though in native Flarum the header is offset using a padding, so it shouldn't "eat" the margin like what happens when two elements with margin top are wrapped in each other.
PS: if the emoji is part of your source code, it also won't work 😛
