Block Editor: Remove the `.is-dark-theme` body class from the admin header.

With the changes to dark theme support in https://github.com/WordPress/gutenberg/pull/28233 to check the real background color of the theme, this no longer serves any purpose.

Follow-up to [44133].

Props scruffian, sabernhardt.
Fixes #52385.
Built from https://develop.svn.wordpress.org/trunk@50142


git-svn-id: http://core.svn.wordpress.org/trunk@49821 1a063a9b-81f0-0310-95a4-ce76da25c4cd

wp-admin/admin-header.php

@@ -192,10 +192,6 @@ $admin_body_class .= ' no-customize-support no-svg';
 if ( $current_screen->is_block_editor() ) {
 	// Default to is-fullscreen-mode to avoid jumps in the UI.
 	$admin_body_class .= ' block-editor-page is-fullscreen-mode wp-embed-responsive';
-
-	if ( current_theme_supports( 'editor-styles' ) && current_theme_supports( 'dark-editor-style' ) ) {
-		$admin_body_class .= ' is-dark-theme';
-	}
 }
 
 $error_get_last = error_get_last();

wp-includes/version.php

@@ -13,7 +13,7 @@
  *
  * @global string $wp_version
  */
-$wp_version = '5.7-alpha-50141';
+$wp_version = '5.7-alpha-50142';
 
 /**
  * Holds the WordPress DB revision, increments when changes are made to the WordPress DB schema.