The easiest way to accomplish what you're trying to do is a combination of some simple javascript (jQuery powered in this case) and CSS3 transitions.
We'll use JS to check for the windows scroll position on every scroll event and compare it to the distance of the bottom of the #main element - if the scroll position is greater, then we'll apply a class to the body to indicate we've scrolled past #main, and then we'll use CSS to define the nav styling for that "state."
So, our basic markup:
<nav class="nav">
<a href="#" class="logo">[logo]</a>
</nav>
<div id="main">#main</div>
<div id="below-main">#below-main</div>
And our javascript:
// get the value of the bottom of the #main element by adding the offset of that element plus its height, set it as a variable
var mainbottom = $('#main').offset().top + $('#main').height();
// on scroll,
$(window).on('scroll',function(){
// we round here to reduce a little workload
var stop = Math.round($(window).scrollTop());
if (stop > mainbottom) {
$('.nav').addClass('past-main');
} else {
$('.nav').removeClass('past-main');
}
});
And, our styles:
.nav {
background-color:transparent;
color:#fff;
transition: all 0.25s ease;
position:fixed;
top:0;
width:100%;
background-color:#ccc;
padding:1em 0;
/* make sure to add vendor prefixes here */
}
.nav.past-main {
background-color:#fff;
color:#444;
}
#main {
height:500px;
background-color:red;
}
#below-main {
height:1000px;
background-color:#eee;
}
A working example on Codepen
This is how I did it here. I also employ some scroll throttling and a bit more complicated styling semantics, but this is the gist of it.
