Q:

A way to only use a CSS class on non mobile devices

So, I need a way to so that a CSS class is only used by desktops (Normal Screen). 
Eg:
.class {
  position: fixed;
}

Is only used on desktops. 

A:

Although it's not desktop specific, you can limit it to the screen based on the screen size with a media rule around it, like so:
@media screen and (min-width: 640px) {
    .class {
        position: fixed;
    }
}

This would limit your rule to only be applied if the monitor was at least 640 pixels wide, larger than a smartphone (today, anyway).