functions.php

<?php
//* Do NOT include the opening php tag

add_filter( 'stylesheet_uri', 'custom_replace_default_style_sheet', 10, 2 );
/**
 * Replace main theme style sheet with custom one if viewing post in a particular category.
 *
 * Note: Replace X with the category ID you want to target.
 */
function custom_replace_default_style_sheet( $stylesheet, $stylesheet_dir ) {

	if ( is_singular( 'post' ) && in_category( X ) ) {
		$stylesheet = $stylesheet_dir . '/custom.css';
	}

	return $stylesheet;

}