MDL-65708 output: Get a base config from Boost

As a part of the removal of bootstrapbase & clean
I removed some logic that got the base config of bootstrapbase,
this should of been changed to get boosts config.
See: e00f1c663c

lib/outputlib.php

@@ -739,7 +739,11 @@ class theme_config {
         $this->name     = $config->name;
         $this->dir      = $config->dir;
 
-        $baseconfig = $config;
+        if ($this->name != self::DEFAULT_THEME) {
+            $baseconfig = self::find_theme_config(self::DEFAULT_THEME, $this->settings);
+        } else {
+            $baseconfig = $config;
+        }
 
         $configurable = array(
             'parents', 'sheets', 'parents_exclude_sheets', 'plugins_exclude_sheets', 'usefallback',