Index: src/wp-content/themes/twentyfifteen/functions.php
===================================================================
--- src/wp-content/themes/twentyfifteen/functions.php	(revision 36701)
+++ src/wp-content/themes/twentyfifteen/functions.php	(working copy)
@@ -73,6 +73,11 @@
 	add_theme_support( 'title-tag' );
 
 	/*
+	 * Enable support to add site logo.
+	 */
+	add_theme_support( 'site-logo', array( 'size' => 'large' ) );
+
+	/*
 	 * Enable support for Post Thumbnails on posts and pages.
 	 *
 	 * See: https://codex.wordpress.org/Function_Reference/add_theme_support#Post_Thumbnails