Fixing Navigation for Mobile Devices in Custom Theme

I inherited a blog with a custom-made theme and I’m trying to adjust the top navigation bar so it will display on one line for mobile devices, as it already does for the full-size. On screens smaller that 736px, the navigation bar is no longer displayed inline and partially hidden. I tried using media queries in the CSS, but they’re not working. Here is the code:

@media screen and (max-width: 320px) {
	#topContainer {
	display: inline;
	overflow: hidden;
	}
}