That's a very bulky way of doing it
At the top of the page you put:
<?$i;?>
Then in index.php you sorround the post with this div:
<div class="<?=($i%2)?"bg1":"bg2";$i++;?>">
In your style sheet you put:
.bg1 {background-color: #FFF;}
.bg2 {background-color: #000;}