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      */ }