Add dark editor style support

This fixes #6451.

This PR enables a new command to themers. They can now add `add_theme_support( 'dark-theme' );` in order to ask the editing canvas to be friendly to dark themes.

What this does is add a body class to the editor, `is-dark-theme`, which enables already present color changes to the side UI and borders, that are friendly to dark themes.

gutenberg.php

@@ -468,6 +468,10 @@ function toggleDropdown() {
  * @return string The $classes string, with gutenberg-editor-page appended.
  */
 function gutenberg_add_admin_body_class( $classes ) {
-	// Default to is-fullscreen-mode to avoid jumps in the UI.
-	return "$classes gutenberg-editor-page is-fullscreen-mode";
+	if ( current_theme_supports( 'editor-styles' ) && current_theme_supports( 'dark-theme' ) ) {
+		return "$classes gutenberg-editor-page is-fullscreen-mode is-dark-theme";
+	} else {
+		// Default to is-fullscreen-mode to avoid jumps in the UI.
+		return "$classes gutenberg-editor-page is-fullscreen-mode";
+	}
 }