Responsive landing page - Grid + Flexbox

First project here. Would love some feedback on code in general, if there are any major red flags in the way I am doing things, that I should cut out now before I get into any bad habits etc. Many thanks in advance :)

One specific thing I was having a little issue with the mobile menu. After the menu has been opened and closed again, and then the screen size adjusted, crossing the breakpoints, the menu will flash on screen and slide out of view, due to the .mobile-menu-close class which contains the animation being present. I had looked at multiple ways of trying to fix this but ended up getting a bit lost. One was using js to remove the class once the screen passed a certain width, but I feel there is probably something very simple which I am overlooking... (I hope that makes sense...)