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<?php
	//Add category class to body on single posts
	add_filter('body_class','add_category_to_single');
	function add_category_to_single($classes, $class) {
		if (is_single() ) {
			global $post;
			foreach((get_the_category($post->ID)) as $category) {
				// add category slug to the $classes array
				$classes[] = 'single-cat-'.$category->category_nicename;
			}
		}
		// return the $classes array
		return $classes;
	}
?>