diff --git a/src/wp-includes/class-wp-customize-control.php b/src/wp-includes/class-wp-customize-control.php
index 9be8e75..1e29989 100644
--- a/src/wp-includes/class-wp-customize-control.php
+++ b/src/wp-includes/class-wp-customize-control.php
@@ -1242,7 +1242,12 @@ class WP_Customize_Theme_Control extends WP_Customize_Control {
 	 * @access public
 	 */
 	public function content_template() {
-		$preview_url = site_url( add_query_arg( 'theme', '{{ data.theme.id }}' ) );
+		$_url = parse_url( admin_url() );
+		$preview_url = $_url['scheme'] . '://' . $_url['host'];
+		if ( $_url['port'] ) {
+			$preview_url .= ':' . $_url['port'];
+		}
+		$preview_url .= add_query_arg( 'theme', '{{ data.theme.id }}' );
 		?>
 		<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] ) { #>