You can use the adjacent sibling combinator:
header+p {
  color: red;
}
<section>
  <header>
    <h2 class="licht"><span id="Standort" class="-sitemap-select-item-selected">Standort</span></h2>
  </header>
  <p>Lorem ipsum</p>
</section>
 
 
If you only can base yourself on the ID of a child element contained in the header, there's no pure CSS solution, and you'd have to rely on JavaScript:
document.getElementById('Standort')
  .closest('header')
  .nextElementSibling
  .style.color = 'red';
<section>
  <header>
    <h2 class="licht"><span id="Standort" class="-sitemap-select-item-selected">Standort</span></h2>
  </header>
  <p>Lorem ipsum</p>
</section>