Child themes

A ready-made theme almost always needs tweaking: change the footer, add a block, alter the colours. If you edit the theme's own files, its very first update erases every change. The right way is a child theme: it takes everything from the parent and stores only the differences.

Creating

Create a folder next to the parent theme, for example wp-content/themes/mytheme-child, and in it a style.css file:

/*
Theme Name: My Theme Child
Template: mytheme
Version: 1.0.0
Text Domain: mytheme-child
*/
 
.site-footer {
    background: #222;
    color: #fff;
}

The Template line is the folder name of the parent theme. It is the only thing that makes a theme a child. Activate it under "Appearance → Themes".

Adding the styles

// functions.php (the child theme)
 
add_action('wp_enqueue_scripts', function () {
    wp_enqueue_style('mytheme-parent', get_template_directory_uri() . '/style.css');
 
    wp_enqueue_style(
        'mytheme-child',
        get_stylesheet_uri(),
        ['mytheme-parent'],
        wp_get_theme()->get('Version'),
    );
});

The child theme's styles load after the parent's and override them.

Overriding a template

To change a template, copy the file from the parent theme into the child with the same name and path — and edit the copy:

mytheme/footer.php              → mytheme-child/footer.php
mytheme/template-parts/card.php → mytheme-child/template-parts/card.php

WordPress looks for a file in the child theme first and only then in the parent. That works for template parts too, provided the parent theme includes them with get_template_part().

functions.php is not replaced

Unlike templates, the child theme's functions.php does not replace the parent's but runs before it. So it holds only additions and changes — through hooks:

// functions.php (the child theme)
 
add_filter('excerpt_length', fn () => 30, 20);
 
add_action('after_setup_theme', function () {
    remove_action('wp_footer', 'mytheme_print_credits');
}, 20);

Priority 20 guarantees the code runs after the parent's code at priority 10.

Two addresses, two folders

When a child theme is not needed

There is no reason to wrap your own theme in a child: nobody else's update will overwrite it. Nor is a child theme needed for a couple of CSS lines — they can go into "Appearance → Customize → Additional CSS". And features unrelated to design — post types, shortcodes — are written in a plugin.

Common mistake

If a function in the parent theme is declared without a function_exists() check, you cannot declare a function of the same name in the child — the result is a fatal "Cannot redeclare" error. Behaviour is changed not by redeclaring but by hooks: take the parent function off with remove_action() or remove_filter() and attach your own.