How to pick CSS file with javascript if -
would possible me like
<link href="nav.css" type="text/css" rel="stylesheet" />
in this?
<script>     if (screen.width <= 800) {         link css here!     } </script>      
just use css media query so
you can read media queries here
@media(max-width: 800px) {     /*      * enter css here      */ }