::selection

p::selection {
  color: red;
  background-color: yellow;
}

<p>
  Select a fragment of this paragraph, to see how its appearance is affected.
</p>

::selection {
  /* ... */
}

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

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

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

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

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