index.phpを変更
<?php if ( have_posts() ) : ?>
<?php twentyeleven_content_nav( 'nav-above' ); ?>
<?php /* Start the Loop */ ?>
<?php while ( have_posts() ) : the_post(); ?>
<?php get_template_part( 'content', get_post_format() ); ?>
<?php endwhile; ?>
↓↓↓
<?php $post_counter = 1; ?>
<?php if ( have_posts() ) : ?>
<?php twentyeleven_content_nav( 'nav-above' ); ?>
<?php /* Start the Loop */ ?>
<?php while ( have_posts() ) : the_post(); ?>
<div class="loop_<?php print "$post_counter"; ?>">
<?php $post_counter++; ?>
<?php get_template_part( 'content', get_post_format() ); ?>
</div>
<?php endwhile; ?>
