WordPress Child Themes for the Working Human

Create a new directory in wp-content/themes — let's call it danktheme in this example. Create two files in that directory: style.css and functions.php with the following contents:

/*
 Theme Name:   Your Themename
 description:  Your Description
 Author:       Your Name
 Template:     twentytwentyone
 Version:      1.0.0
*/

and

<?php
add_action('wp_enqueue_scripts', 'my_theme_enqueue_styles', 11);
function my_theme_enqueue_styles()
{
    wp_enqueue_style('danktheme', get_stylesheet_uri());
}

That's it: now you can select "Your Themename" in the admin interface. If you don't want to use twentytwentyone as the parent theme — because it's not the most recent one when you read this, for example — replace it in the style.css. Yes, in the stylesheet, I'm not making this up, rules are rules.