How to modify an item at a specific point on the page? (HTML, CSS)

Does it have to be only with CSS?

If it can be with JavaScript you can use jQuery scroll touchmove .

Here's an example:

$(window).on("scroll touchmove", function() {
  if ($(document).scrollTop() >= $("#mudar").position().top) {
    $('body').css('background-color', 'orange');
  } else {
    $('body').css('background-color', 'white');
  }
});

div{
  height: 300px;
  padding: 10px;
}

<script src="https://ajax.googleapis.com/ajax/libs/jquery/3.2.1/jquery.min.js"></script><div>Nãomuda.</div><div>Nãomuda.</div><div>Nãomuda.</div><divid="mudar">Mudar quando chegar aqui.</div>
<div>Não muda.</div>
<div>Não muda.</div>
<div>Não muda.</div>
<div>Não muda.</div>
<div>Não muda.</div>
<div>Não muda.</div>

  

The advantage of this scroll touchmove function is that you do not need   set the size of the scroll that will be made to change the   element property, because the size of the scroll may change depending on the   device that is accessing the site. So the scroll touchmove only   change the property when the top of the browser arrives at the element   chosen that in the case of my code it was <div> with id="mudar" .

I learned from the guy who answered this question of mine: Change appearance by going through the section

You can use both jQuery and Javascript.

JavaScript

window.addEventListener('scroll', myFunction);

function myFunction() {
  // Seleciona o elemento alvo
  var element = document.querySelector(".element");
  if (window.pageYOffset > 100) {
    // Ao descer 100px, realiza as modificações desejadas
    element.classList.add('scrolled');
  } else {
    // Ao retornar ao topo da página, desfaz as modificações
    element.classList.remove('scrolled');
  }
}

jQuery

$(window).scroll(function(e){
  if ($(this).scrollTop() > 100) {
    // Ao descer 100px, realiza as modificações desejadas
    $('.element').addClass('scrolled');
  } else {
    // Ao retornar ao topo da página, desfaz as modificações
    $('.element').removeClass('scrolled');
    }
});

Examples

• JavaScript
• jQuery