activity loop:
`	<?php if ( empty( $_POST['page'] ) ) : ?>
		<ul id="activity-stream" class="activity-list item-list">
	<?php endif; ?>

	<?php /* Line above */ ?>
		<?php $zebra_class = 'even'; ?>

	<?php while ( bp_activities() ) : bp_the_activity(); ?>

	<?php /* Line below */ ?>
<?php $zebra_class = ($zebra_class == 'even') ? 'odd' : 'even'; ?>

		<?php include( locate_template( array( 'activity/entry.php' ), false ) ) ?>`

entry:
`<?php /* print out the zebra class */ ?>
<li class="<?php bp_activity_css_class() ?><?php print $zebra_class; ?>" id="activity-<?php bp_activity_id() ?>">`

css:
`div.activity ul.item-list li:nth-child(odd) { background: #F4F4F4; }`