Changing z-index value of navigation menu?

I am using a map plugin that is covering the menu bar when I scroll up here. To solve this I think I need to increase the z-index value. I tried to do this via customizer - additional CSS. the value in the Theme CSS is 100, so I added:

.main-navigation {
	z-index: 500;
	padding: 0;
	clear: both;
	display: block;
}

.sticky-navigation {
	z-index: 500;
	padding: 0;
	clear: both;
	display: block;
}

(I did both as I'm not sure which one I need to do...).

This doesn't seem to have changed anything. What can I do?