Add CSS Class To Nav When User Scrolls

This is from the StudioPress Altitude Pro theme. When user is at the top of the page, the site-header is transparent. When the user scrolls the page, the ‘dark’ class is added to the site-header div, allowing you to style the .site-header.dark element as you see fit.

jQuery(function( $ ){
	if( $( document ).scrollTop() > 0 ){
		$( '.site-header' ).addClass( 'dark' );			
	}
	// Add opacity class to site header
	$( document ).on('scroll', function(){
		if ( $( document ).scrollTop() > 0 ){
			$( '.site-header' ).addClass( 'dark' );			
		} else {
			$( '.site-header' ).removeClass( 'dark' );			
		}
	});
});