document.addEventListener('DOMContentLoaded', function(){

    var mobile = window.matchMedia( 'only screen and ( max-width: 640px )');

    applyMobileStyles()

    mobile.addListener( applyMobileStyles );

    function applyMobileStyles() {
        if ( mobile.matches ) {
            document.body.style.backgroundColor = 'grey';
            console.log('mobile');
        } else {
            document.body.style.backgroundColor = 'grey';
        }
    }
})