Fixed child theme compatibility

Many thanks for the help on your forum, this pull request should fix the fault occurring for any of the other files. 

Allows child themes to override the following files (will work now) 

gavern/form_elements/*.php and *.css
gavern/form.parser.php
gavern/layouts/update.php 
css/backend/*.css (for custom admin styles via child theme) 
js/backend/*.js (custom admin JS via child theme)