html {
	font-size: 10px;
}

@media only screen and (max-width: 60rem) {
	body {
		color: #fff;
		background: #333;
	}
}
	

The media query should trigger at 960px. When resizing the window (or using responsive mode), it is triggered at 600px. Tested in Firefox 58.0.2 on macOS 10.12.6.