Twenty Eleven – Overwrite the Dark Color Scheme in a Child Theme

Overwriting or customizing the dark color schem in a child theme of Twenty ten is difficult, because the dark stylesheet /colors/dark.css get loaded after the style.css of the child theme.
You can duplicate the same stylesheet structure in the child theme, but you need to programm the child theme to link the /colors/dark.css file after the corresponding file of the parent theme.

The stylesheet dark.css of the parent theme gets enqueued in twentyeleven/inc/theme-options.php (line 319):

 * Enqueue the styles for the current color scheme.
 * @since Twenty Eleven 1.0

Therefore, the dark.css of the child theme needs to be enqueued with a different priority; which can be done by adding a corresponding, adapted code into functions.php of your child theme. This code checks if the ‘dark’ color scheme is selected, and only then enqueues the /colors/dark.css of the child theme, thus allowing full color control from the theme options in the dashboard:

/** Enqueue the stylesheet for the current color scheme into the child theme. */
function twentyelevenchild_enqueue_color_scheme() {
	$options = twentyeleven_get_theme_options();
	$color_scheme = $options['color_scheme'];
	if ( 'dark' == $color_scheme )
		wp_enqueue_style( 'dark_child', get_stylesheet_directory_uri() . '/colors/dark.css', array(), null );
	do_action( 'twentyelevenchild_enqueue_color_scheme', 'dark_child' );
add_action( 'wp_enqueue_scripts', 'twentyelevenchild_enqueue_color_scheme', 11);

Now you can edit dark.css in your child theme, and add all the customisations to it.