functions.php

<?php
/**
 * Plain Notebook functions and definitions.
 *
 * @link https://developer.wordpress.org/themes/basics/theme-functions/
 *
 * @package Plain_Notebook
 */

if ( ! function_exists( 'plain_notebook_setup' ) ) :
/**
 * Sets up theme defaults and registers support for various WordPress features.
 *
 * Note that this function is hooked into the after_setup_theme hook, which
 * runs before the init hook. The init hook is too late for some features, such
 * as indicating support for post thumbnails.
 */
function plain_notebook_setup() {

	// Add default posts and comments RSS feed links to head.
	add_theme_support( 'automatic-feed-links' );

	/*
	 * Let WordPress manage the document title.
	 * By adding theme support, we declare that this theme does not use a
	 * hard-coded <title> tag in the document head, and expect WordPress to
	 * provide it for us.
	 */
	add_theme_support( 'title-tag' );

	/*
	 * Enable support for Post Thumbnails on posts and pages.
	 *
	 * @link https://developer.wordpress.org/themes/functionality/featured-images-post-thumbnails/
	 */
	add_theme_support( 'post-thumbnails' );

	// This theme uses wp_nav_menu() in one location.
	register_nav_menus( array(
		'primary' => esc_html__( 'Primary', 'plain-notebook' ),
	) );

}
endif;
add_action( 'after_setup_theme', 'plain_notebook_setup' );

/**
 * Set the content width in pixels, based on the theme's design and stylesheet.
 *
 * Priority 0 to make it available to lower priority callbacks.
 *
 * @global int $content_width
 */
function plain_notebook_content_width() {
	$GLOBALS['content_width'] = apply_filters( 'plain_notebook_content_width', 540 );
}
add_action( 'after_setup_theme', 'plain_notebook_content_width', 0 );

/**
 * Enqueue scripts and styles.
 */
function plain_notebook_scripts() {
	wp_enqueue_style( 'plain-notebook-style', get_stylesheet_uri() );

	if ( is_singular() && comments_open() && get_option( 'thread_comments' ) ) {
		wp_enqueue_script( 'comment-reply' );
	}
}
add_action( 'wp_enqueue_scripts', 'plain_notebook_scripts' );