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<?php

function ccd_add_category_to_single( $classes, $class ) {
	if ( is_single() ) {
		global $post;
		foreach( ( get_the_category( $post->ID ) ) as $category ) {
			$classes[] = 'single-cat-'.$category->category_nicename;
		}
	}
	return $classes;
}
add_filter( 'body_class','ccd_add_category_to_single' );

?>