Index: src/wp-includes/class-wp-customize-control.php
===================================================================
--- src/wp-includes/class-wp-customize-control.php	(revision 32077)
+++ src/wp-includes/class-wp-customize-control.php	(working copy)
@@ -1242,7 +1242,8 @@
 	 * @access public
 	 */
 	public function content_template() {
-		$preview_url = site_url( add_query_arg( 'theme', '{{ data.theme.id }}' ) );
+		$current_url = set_url_scheme( 'http://' . $_SERVER['HTTP_HOST'] . $_SERVER['REQUEST_URI'] );
+		$preview_url = add_query_arg( 'theme', '{{ data.theme.id }}', $current_url );
 		?>
 		<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] ) { #>