Can't get editor-style.css to load at all

I’m trying to add some custom css styling to my editor, but I just can’t get editor-style.css to load at all.

The css file I’m trying to use is called editor-style.css, and it’s in my theme folder.

I’ve tried adding this to functions.php:

add_editor_style();

That didn’t work, I also tried this:

function mytheme_add_editor_style() {
    add_editor_style();
}
add_action( 'init', 'mytheme_add_editor_style' );

That didn’t work either. When I load the editor page, I’m using Chrome’s F12 debugging view to see what stylesheets are being loaded, and editor-style.css isn’t there at all.

If it’s relevant, this is in a child theme.

What am I doing wrong?