Add category or categories slug(s) as class(es) to a category archive or single post as answered on Stackexchange question https://wordpress.stackexchange.com/questions/253677/different-body-classes-for-each-category/253708#253708

add_category_to_body_class.php

// Adding categories name to body class on archives
add_filter('body_class','add_category_to_classes');

function add_category_to_classes( $classes ) {
	if ( is_single() ) {
		global $post;
		// assigns post it to variable to avoid reading an array for every loop iteration
		// maybe a micro optmization? Guilty as charged!
		$post_id = $post->ID;
		foreach( ( get_the_category( $post_id ) ) as $category ) {
			// add category slug to the $classes array
			$classes[] = $category->category_nicename;
			$classes[] = $category->category_nicename . '_index';
		}
	}
	// check if in category
	else if ( is_category() ) {
		// get the queried object, in this case, a category object, and assigns to a variable
		// see https://codex.wordpress.org/Function_Reference/get_queried_object
		$category_object = get_queried_object();
		// get the slug
		$classes[] = $category_object->slug;
		$classes[] = $category_object->slug . '_index';
	}
	else {
		$classes[] = 'index';
	}
	// return the $classes array
	return $classes;
}