Index: src/wp-content/themes/twentyfourteen/functions.php
===================================================================
--- src/wp-content/themes/twentyfourteen/functions.php	(revision 39637)
+++ src/wp-content/themes/twentyfourteen/functions.php	(working copy)
@@ -545,3 +545,17 @@
 if ( ! class_exists( 'Featured_Content' ) && 'plugins.php' !== $GLOBALS['pagenow'] ) {
 	require get_template_directory() . '/inc/featured-content.php';
 }
+
+/**
+ * Add an `is_customize_preview` function if it is missing.
+ *
+ * Enables installing Twenty Fourteen in WordPress versions before 4.0.0 when the
+ * `is_customize_preview` function was introduced.
+ */
+if ( ! function_exists( 'is_customize_preview' ) ) :
+function is_customize_preview() {
+	global $wp_customize;
+
+	return ( $wp_customize instanceof WP_Customize_Manager ) && $wp_customize->is_preview();
+}
+endif;