MDL-65438 core_themes: allow themes to change the css url

Themes that have the function theme_themename_alter_css_urls() in lib.php
can change the css url from theme/styles.php to a local script like
theme/themename/css.php. The local css.php file can be used to serve
various versions of the theme css, for example versions compiled using
different scss variables.

lib/outputlib.php

@@ -1103,6 +1103,8 @@ public function css_urls(moodle_page $page) {
             }
         }
 
+        // Allow themes to change the css url to something like theme/mytheme/mycss.php.
+        component_callback('theme_' . $this->name, 'alter_css_urls', [&$urls]);
         return $urls;
     }