MDL-56959 theme: SCSS file is inherited from parent

Also added support for the SCSS property to be a Closure
and return content rather than simply being the name of a file.

theme/boost/config.php

@@ -27,9 +27,11 @@ defined('MOODLE_INTERNAL') || die();
 require_once(__DIR__ . '/lib.php');
 
 $THEME->name = 'boost';
-$THEME->scssfile = 'preset';
 $THEME->sheets = [];
 $THEME->editor_sheets = [];
+$THEME->scss = function($theme) {
+    return theme_boost_get_main_scss_content($theme);
+};
 
 $THEME->layouts = [
     // Most backwards compatible layout without the blocks - this is the layout used by default.