Add categories to body class in WordPress

add_body_class.php

<?php
function _add_body_class($classes, $custom_classes = false)
{
    the_post();
    
    // insert buddypress class on buddypress page
    if ( function_exists("bp_is_blog_page") && !bp_is_blog_page() )
        $classes[] = 'buddypress';
    
    // add category
    if ( is_single() ) {
        foreach (get_the_category() as $category) {
            $classes[] = "single-".$category->slug;
        }
    }
    
    rewind_posts();
    
    return $classes;
}
add_filter("body_class", "_add_body_class", 10, 2 );