SCSS

HTML

<main>
  <div class="promo">
    <p>
    stuff here
    </p>
  </div>
</main>

<footer class="my-footer">
  <div class="footer-inner">
  <p>
  change background of this div from promo
  </p>
  </div>
</footer>

SCSS

.footer-inner {
  background: red;
  padding: 20px;
}

.promo > .footer-inner,
.promo + .footer-inner,
.promo ~ .footer-inner {
  background: yellow !important;
}