JavaScript Snippet

        <script src="https://code.jquery.com/jquery-3.6.0.min.js"></script>
    <script>
    var offset = 400
    $(window).on('load scroll', function(){
        
        if( $(window).scrollTop() > offset ){
            $('body').addClass('show')
        }else{
            $('body').removeClass('show')
        }
    })
    </script>

CSS Snippet

selector{
    opacity: 0;
    transition: all 0.3s ease-in-out;
}
body.show selector{
    opacity: 1;
}