theme_setup_functions.php

function theme_setup() {
    add_theme_support('post-thumbnails'); // adds thumbnail support for the pages, posts and Projects CPT
    add_image_size('work_cat_thumb', 800, 500, true);
    add_image_size('post_thumbnail', 600, 250, true);
    add_image_size('post_thumbnail_1', 70, 70, true);


// Register menus

    register_nav_menus(array(
            'frontpage-menu' => __('Front Page Menu', 'theme')
        )
    );
    theme_default_menu();
}

add_action('after_setup_theme', 'theme_setup');

/**
 * function to setup default theme menu
 */
function theme_default_menu() {

    $menuname = 'One Front Page Menu';
    $menulocation = 'frontpage-menu';
}