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
 */
...etc...

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.

Twenty Eleven – Sidebar on Single Posts and Pages

The new default theme of WP3.2 – Twenty Eleven – is a very versatile theme.
However, the ‘missing’ sidebar on single posts or pages is quite disturbing for some users, loosing their advertising space or the main navigation.
To get the sidebar back is not that simple – style.css is quite complex, and the layout of a single post or page is done with a clever use of a specially introduced css class in the body tag.

First step

Create a child theme.

– create a new sub-folder under the /wp-content/themes folder, for instance: twentyelevenchild;
– add a style.css file to that folder; content:

/*
Theme Name: Twenty Eleven Child
Author: alchymyth
Description: a child theme, based on the 2011 theme for WordPress
Author URI: http://wordpress.org/
Template: twentyeleven
*/

@import url(../twentyeleven/style.css);

– add a functions.php file to that folder; to be used later;
– optional: add a screenshot.jpg image to that folder, depicting the design of your child theme.

Second step

Add the call of the sidebar back.

Edit single.php and/or page.php and add

<?php get_sidebar(); ?>

before the line

<?php get_footer(); ?>

Third step

Suppress the .singular body_class from the single post or page.

To achieve this, add a filter to your new functions.php in your child theme folder.

add_filter('body_class', 'blacklist_body_class', 20, 2);
function blacklist_body_class($wp_classes, $extra_classes) {
if( is_single() || is_page() ) :
// List of the classes to remove from the WP generated classes
$blacklist = array('singular');
// Filter the body classes
  foreach( $blacklist as $val ) {
    if (!in_array($val, $wp_classes)) : continue;
    else:
	  foreach($wp_classes as $key => $value) {
	  if ($value == $val) unset($wp_classes[$key]);
	  }
    endif;
  }
endif;   // Add the extra classes back untouched
return array_merge($wp_classes, (array) $extra_classes);
}

(resources:
http://dev-tips.com/featured/remove-an-item-from-an-array-by-value
http://wordpress.stackexchange.com/questions/15850/remove-classes-from-body-class )

Fourth step

Fine-tuning of style.css

For instance:

.single #author-info {
	background: #f9f9f9;
	border-top: 1px solid #ddd;
	border-bottom: 1px solid #ddd;
	margin: 2.2em 0% 0 0%;
	padding: 20px 35.4%;
}

There might be some more details which can be changed while customizing the styles of the child theme.

edit: thanks to member smijos of the wordpress support forum:
some more styles, for the comment section, to make the theme look good:

#respond {
width: auto;
}
.commentlist {
width: auto;
}
.commentlist > li.comment {
margin: 0px 0px 20px 102px;
width: auto;
}

PS: A Few Different Cases

(updated and expanded)

this all concerns this line of code:

if( is_single() || is_page() ) :

1) if you rather use the sidebar-page template that comes with the theme, to control if and when to show a sidebar on a static page, simply don’t edit page.php, and remove

|| is_page()

from that line.

2) Zeaks pointed out that the code also effects single attachment pages.

if you want the sidebar in these pages, edit image.php and add

<?php get_sidebar(); ?>

before the ‘get_footer’ line; also adapt the style of

.image-attachment div.attachment

.

if you want your single attachment pages without sidebar, change to:

if( (is_single() && !is_attachment()) || is_page() ) :

PPS

If you don’t like coding, please do still create the child theme; however, for the rest, there is a plugin available:
‘Twenty Eleven Theme Extensions’