style.css

.even {
  background:#f0f8ff;  
} 

.odd {
  background:#f4f4fb;
}

wp_odd_even_classes_posts.php

<?php

  function oddeven_post_class ( $classes ) {
     global $current_class;
     $classes[] = $current_class;
     $current_class = ($current_class == 'odd') ? 'even' : 'odd';
     return $classes;
  }
  add_filter ( 'post_class' , 'oddeven_post_class' );
  global $current_class;
  $current_class = 'odd';

?>