Ticket #28637: 28637.2.diff

src/wp-includes/theme.php

@@ -953 +953 @@
          * @param string $old_value The current value of the theme mod.
          */
         $mods[ $name ] = apply_filters( "pre_set_theme_mod_$name", $value, $old_value );
-
+        
+        // If the new value is empty, remove it entirely so that get_theme_mod() returns false
+        if ( '' === $value ) {
+                unset( $mods[ $name ] );
+        }
+        
         $theme = get_option( 'stylesheet' );
         update_option( "theme_mods_$theme", $mods );
 }