MDL-77657 theme: Fix get_extra_scss_code load order

This led to problems with grandchild themes,
depending on their direct parent's extrascsscallback.

lib/outputlib.php

@@ -1598,7 +1598,7 @@ public function get_extra_scss_code() {
 
         // Getting all the candidate functions.
         $candidates = array();
-        foreach ($this->parent_configs as $parent_config) {
+        foreach (array_reverse($this->parent_configs) as $parent_config) {
             if (!isset($parent_config->extrascsscallback)) {
                 continue;
             }