The Mobile First Approach
When people say "mobile first" in referring to web design & theming, it's not just a mentality or something people say in a "the way it ought to be" sort of sense. They literally mean, designing the mobile theme first, then adding on CSS for tablets, desktop, and wide desktop themes. In a technical sense, it means that the global.css that contains the basic design elements of the site is loaded no matter what device you are using to view the website. Then, CSS3 Media queries determine what the size of the window is and additional stylesheets will be tacked on; either "narrow", "normal" , or "wide" - depending on the width of the device.