Theme override for field acces field select widget to add an additional class.

template.php

<?php 

/**
 * Override theme_faf_select().
 */
function mytheme_faf_select($element) {
  $children = $element['#children'];
  // Adds a faf-field class to the first appearance of the form-item class.
  $children = preg_replace('&class="[^"]*(form-item)[^"]*"&is', 'class="\1 faf-select"', $children, 1);
  return $children;
}