How to add additional post classes

functions.php

<?php

add_filter( 'post_class', 'custom_post_classes' );
/**
 * Add a class of "top" to the first post, and "odd"/"even" classes as appropriate.
 *
 * @global int $loop_counter Counts the number of times we do the loop.
 *
 * @param array $classes Existing array of post classes.
 * 
 * @return array Amended post classes array.
 */
function custom_post_classes($classes) {

    global $loop_counter;

    if (0 == $loop_counter) // First post
        $classes[] = 'top';

    if ($loop_counter % 2) // Loop counter is exactly divisible by 2
        $classes[] = 'even';
    else
        $classes[] = 'odd';

    return $classes;

}

style.css

#content .post { // This isn't a custom class
    background-color: #fff;
}
#content .odd {
    background-color: #fdfdfd;
}