Index: src/wp-includes/class-wp-customize-control.php
===================================================================
--- src/wp-includes/class-wp-customize-control.php	(revision 32255)
+++ src/wp-includes/class-wp-customize-control.php	(working copy)
@@ -1245,6 +1245,9 @@
 		$current_url = set_url_scheme( 'http://' . $_SERVER['HTTP_HOST'] . $_SERVER['REQUEST_URI'] );
 		$preview_url = esc_url( add_query_arg( 'theme', '__THEME__', $current_url ) ); // Token because esc_url() strips curly braces.
 		$preview_url = str_replace( '__THEME__', '{{ data.theme.id }}', $preview_url );
+
+		// Doesn't work, because $this->theme is empty
+		$is_active_theme = $this->theme->stylesheet === $this->manager->original_stylesheet;
 		?>
 		<div class="theme" tabindex="0" data-preview-url="<?php echo esc_attr( $preview_url ); ?>" aria-describedby="{{ data.theme.id }}-action {{ data.theme.id }}-name">
 			<# if ( data.theme.screenshot[0] ) { #>
Index: src/wp-includes/class-wp-customize-manager.php
===================================================================
--- src/wp-includes/class-wp-customize-manager.php	(revision 32255)
+++ src/wp-includes/class-wp-customize-manager.php	(working copy)
@@ -33,7 +33,7 @@
 	 *
 	 * @var string
 	 */
-	protected $original_stylesheet;
+	public $original_stylesheet;
 
 	/**
 	 * Whether this is a Customizer pageload.
@@ -1198,9 +1198,25 @@
 		require_once( ABSPATH . 'wp-admin/includes/theme.php' );
 
 		// Theme Controls.
+
+		if ( ! $this->is_theme_active() ) {
+			/*
+			 * Add a control for the active/original theme.
+			 * The first array element of wp_prepare_themes_for_js() is empty,
+			 * because it's reserved for the current theme. But we want
+			 * prepared data for just one theme...
+			 */
+			$active_theme = current( array_filter( wp_prepare_themes_for_js( array( wp_get_theme( $this->original_stylesheet ) ) ) ) );
+			$this->add_control( new WP_Customize_Theme_Control( $this, $active_theme['id'], array(
+				'theme'    => $active_theme,
+				'section'  => 'themes',
+				'settings' => 'active_theme',
+			) ) );
+		}
+
 		$themes = wp_prepare_themes_for_js();
 		foreach ( $themes as $theme ) {
-			if ( $theme['active'] ) {
+			if ( $theme['active'] || $theme['stylesheet'] === $this->original_stylesheet ) {
 				continue;
 			}