Overlay

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This content will appear a dark overlay background. 
  <div class="overlay">
    <div class="overlay--content">
      This content will appear a dark overlay background.
    </div>
  </div>

Combine the overlay mixin with background-cover for a nice darkened image effect:

This content will appear over a darkened image 
  <div class="example-overlay">
    <div class="overlay--content">
      This content will appear over a darkened image
    </div>
  </div>