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.