::selection

::selection {
  color: gold;
  background: red;
}

/* Firefox syntax */
::-moz-selection

::selection

<div>This text has special styles when you highlight it.</div>
<p>Also try selecting text in this paragraph.</p>

/* Make selected text gold on a red background */
::-moz-selection {
  color: gold;
  background: red;
}

::selection {
  color: gold;
  background: red;
} 

/* Make selected text in a paragraph white on a blue background */
p::-moz-selection {
  color: white;
  background: blue;
}

p::selection {
  color: white;
  background: blue;
}