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)
@@ -1268,6 +1268,67 @@
 }
 
 /**
+ * Customize Active Theme Control class.
+ *
+ * @since 4.2.0
+ *
+ * @see WP_Customize_Control
+ */
+class WP_Customize_Active_Theme_Control extends WP_Customize_Control {
+
+	/**
+	 * Customize control type.
+	 *
+	 * @since 4.2.0
+	 * @access public
+	 * @var string
+	 */
+	public $type = 'active_theme';
+
+	/**
+	 * Theme object.
+	 *
+	 * @since 4.2.0
+	 * @access public
+	 * @var WP_Theme
+	 */
+	public $theme;
+
+	/**
+	 * Render control's content from PHP.
+	 *
+	 * @since 4.2.0
+	 * @access public
+	 */
+	public function render_content() {
+		$current_url = set_url_scheme( 'http://' . $_SERVER['HTTP_HOST'] . $_SERVER['REQUEST_URI'] );
+		$preview_url = esc_url( remove_query_arg( 'theme', $current_url ) );
+		$aria_action = esc_attr( $this->theme['id'] . '-action' );
+		$aria_name   = esc_attr( $this->theme['id'] . '-name' );
+		?>
+		<div class="theme active" tabindex="0" data-preview-url="<?php echo esc_attr( $preview_url ); ?>"  aria-describedby="<?php echo $aria_action . ' ' . $aria_name; ?>">
+			<?php if ( ! empty( $this->theme['screenshot'][0] ) ) { ?>
+				<div class="theme-screenshot">
+					<img src="<?php echo $this->theme['screenshot'][0]; ?>" alt="" />
+				</div>
+			<?php } else { ?>
+				<div class="theme-screenshot blank"></div>
+			<?php } ?>
+			<span class="more-details" id="<?php echo $aria_action; ?>"><?php _e( 'Customize' ); ?></span>
+			<div class="theme-author"><?php printf( __( 'By %s' ), $this->theme['author'] ); ?></div>
+
+			<h3 class="theme-name" id="<?php echo $aria_name; ?>">
+				<?php
+				/* translators: %s: theme name */
+				printf( __( '<span>Active:</span> %s' ), $this->theme['name'] );
+				?>
+			</h3>
+		</div>
+		<?php
+	}
+}
+
+/**
  * Widget Area Customize Control class.
  *
  * @since 3.9.0
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)
@@ -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_Active_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;
 			}