Index: theme.php
===================================================================
--- theme.php	(revision 33415)
+++ theme.php	(working copy)
@@ -951,6 +951,20 @@
 	if ( is_string( $default ) )
 		$default = sprintf( $default, get_template_directory_uri(), get_stylesheet_directory_uri() );
 
+	/**
+	 * Filter the theme modification default value.
+	 *
+	 * The dynamic portion of the hook name, `$name`, refers to
+	 * the key name of the modification array. For example,
+	 * 'header_textcolor', 'header_image', and so on depending
+	 * on the theme options.
+	 *
+	 * @since 4.3.0
+	 *
+	 * @param string $default The default value of the current theme modification.
+	 */
+	$default = apply_filters( "theme_mod_{$name}_default", $default );
+
 	/** This filter is documented in wp-includes/theme.php */
 	return apply_filters( "theme_mod_{$name}", $default );
 }