The first time I fixed an existing site, I edited the theme's header.php directly. A week later the client updated the theme — every change was gone. That's how I learned about child themes.

What is a child theme?

A theme that inherits everything from a parent theme. You only put what you change into the child; the rest comes from the parent, which can update safely.

Two files

Create wp-content/themes/your-theme-child/ with:

style.css

/*
Theme Name: Your Theme Child
Template: your-theme
Version: 1.0.0
*/

Template must match the parent theme's folder name.

functions.php

<?php
add_action( 'wp_enqueue_scripts', function () {
    wp_enqueue_style(
        'child-style',
        get_stylesheet_uri(),
        [],
        wp_get_theme()->get( 'Version' )
    );
} );

Activate it under Appearance → Themes. If the parent's CSS isn't loaded automatically, enqueue get_template_directory_uri() . '/style.css' first.

Overriding templates

Copy a file such as footer.php into the child theme (same relative path) and edit it; WordPress prefers the child's copy. Your copy won't receive the parent's future updates — override only when needed and prefer hooks.

functions.php is different

The child's functions.php doesn't replace the parent's — it loads before it and both run. Don't redeclare functions the parent defines.

Block themes

Block themes support child themes too: templates are .html files in templates/ and parts/, configured by theme.json. Changes made in the Site Editor live in the database; export them if you want them in code.

When you don't need one

• When you build a custom theme for the project — it's already yours.
• When you only need a few CSS lines — Additional CSS is enough.