Setting custom CSS for an element

Use “selector” in the Advanced > Custom CSS box

selector {
  opacity: 0.7;
}

selector a {
  font-weight: bold;
}

Changing based on screen width

Elementor often applies your parameters like padding a div down from the flex container you are setting them on when it comes to displaying the page. If you apply @media settings for a smaller screen can have no effect as a result. An easy solution is set it for the worst case in elementor settings, then apply your change for a wider screen in the custom CSS, like this example of give me 100px padding if the screen is wide enough:

@media (min-width: 1601px)
{
  selector
  {
    padding-right: 80px !important;
  }
}