:nth-last-child

/* Selects every fourth child element inside the body */
/* regardless of element type */
/* counting backwards from the last one */
body :nth-last-child(4n) {
  background-color: lime;
}

:nth-last-child( <nth> [ of <selector># ]? )where 
<nth> = <an-plus-b> | even | odd

<table>
  <tbody>
    <tr>
      <td>First line</td>
    </tr>
    <tr>
       <td>Second line</td>
    </tr>
    <tr>
       <td>Third line</td>
    </tr>
    <tr>
       <td>Fourth line</td>
    </tr>
    <tr>
       <td>Fifth line</td>
    </tr>
  </tbody>
</table>

table {
  border: 1px solid blue;
}

/* Select the last three elements */
tr:nth-last-child(-n+3) {
  background-color: lime;
}