Inject class into $before_widget variable

gistfile1.php

// Let's inject a little class into our $before_widget variable
if( strpos($before_widget, 'class') === false ) {                 // It has no class... let's add it!
  $before_widget = str_replace('>', ' class="custom-class-name">', $before_widget);
} else {                                                          // We have class... let's append it!
  $before_widget = str_replace('class="', 'class="custom-class-name ', $before_widget);
}