$(window).on('scroll', function(){

    // in percentage
    //$('#custom-scrollbar-thumb').css('top', -$(window).scrollTop() / ($(window).height() - $('body')[0].scrollHeight) * 100 +'%' );

    // in pixels
    $('#custom-scrollbar-thumb').css('top', $(window).scrollTop() * ($(window).scrollTop() / $('body')[0].scrollHeight) +'px' );

});

$(window).on('resize', function(){

    $('#custom-scrollbar-thumb').css('height', window.innerHeight * (window.innerHeight / document.body.offsetHeight) +'px' );

});