Change CSS element with JQuery when scroll reaches an anchor point

I currently have this solution to change the css elements when the page reaches a certain point but I'd like to use an #anchor-point instead of the pixel value (1804) to be responsive on smaller screens. I know it must be easy but I can't find how:

$(document).scroll(function(){
    if($(this).scrollTop() > 1804)
    {   
        $('#voice2').css({"border-bottom":"2px solid #f4f5f8"});
        $('#voice3').css({"border-bottom":"2px solid #2e375b"});
    }
});

This is the current state: http://tibio.ch Thank you,

Try this:

var targetOffset = $("#anchor-point").offset().top;

var $w = $(window).scroll(function(){
    if ( $w.scrollTop() > targetOffset ) {   
        $('#voice2').css({"border-bottom":"2px solid #f4f5f8"});
        $('#voice3').css({"border-bottom":"2px solid #2e375b"});
    } else {
      // ...
    }
});

$(window).bind("scroll", function() { 
   var $sec1 = $('.bg1').offset().top;
     var $sec2 = $('.bg2').offset().top;
     var $sec3 = $('.bg3').offset().top;
   var $sec4 = $('.bg4').offset().top; 
   var $sec5 = $('.carousel-indicators').offset().top;   

   if ($(this).scrollTop() < $sec2){ 
     $(".navbar1").fadeOut();  
     $(".navbar2").fadeOut();  
     $(".navbar3").fadeOut();  
   }     
   if ($(this).scrollTop() > $sec2 & $(this).scrollTop() < $sec3){ 
     $(".navbar1").fadeIn();   
     $(".navbar2").fadeOut();      
   } 
   if ($(this).scrollTop() > $sec3 & $(this).scrollTop() < $sec4){ 
     $(".navbar2").fadeIn();  
     $(".navbar3").fadeOut();  
   }    
   if ($(this).scrollTop() > $sec4 & $(this).scrollTop() < $sec5){ 
     $(".navbar3").fadeIn();  
     $(".navbar2").fadeOut();  
   }    
   if ($(this).scrollTop() > $sec5){ 
     $(".navbar1").fadeOut();  
     $(".navbar2").fadeOut();  
     $(".navbar3").fadeOut();   
   }         
 });

function scroll_style() {
   var window_top = $(window).scrollTop();
   var div_top = $('#anchor-point').offset().top;

   if (window_top > div_top){
      $('#voice2').css({"border-bottom":"2px solid #f4f5f8"});
      $('#voice3').css({"border-bottom":"2px solid #2e375b"});
   }
}
$(function() {
  $(window).scroll(scroll_style);
  scroll_style();
 });

Solution based on: http://blog.yjl.im/2010/01/stick-div-at-top-after-scrolling.html