snippet.php

<?php
/*
 * Use Parent Category
 * Description: Causes subcategories to use their parent's template
*/
function load_cat_parent_template(){
    global $wp_query;

    if (!$wp_query->is_category) {
        return true; // saves a bit of nesting
    }

    // get current category object
    $cat = $wp_query->get_queried_object();

    // trace back the parent hierarchy and locate a template
    while ($cat && !is_wp_error($cat)) {
        $template = get_template_directory() . "/category-{$cat->slug}.php";

        if (file_exists($template)) {
            load_template($template);
            exit;
        }
        $cat = $cat->parent ? get_category($cat->parent) : false;
    }
}
add_action('template_redirect', 'load_cat_parent_template');