Edit lib/outputlib.php and add a 10 second sleep to the end of the function "get_css_content()", right before the "return" statement: public function get_css_content() { ... sleep(10); return $csscontent; } Login as admin Navigate to " Site administration / Appearance / Themes / Theme settings " Make sure the setting "Theme designer mode" is disabled. Go to: /admin/settings.php?section=themesettingboost Change the "Brand color" to something garish (pink works!).  Save the changes. Verify you see (garish colour) text here and there. From the command line, run these commands in your moodle root folder: php admin/cli/purge_caches.php   php admin/cli/build_theme_css.php Now change to your moodledata folder and run this command: rm -rf localcache Now reload the page in the web browser. Confirm that it reloads "quickly" (we are checking that we did not have to rebuild the CSS). Confirm that the page still shows the "garish" text colour (we are checking we didn't use the fallback CSS). Using your file browser, navigate to the moodledata folder. Confirm that the " localcache " folder has re-appeared. Open the localcache folder. Confirm that it contains a "theme" subdirectory (we are checking the CSS was promoted from MUC to localcache).