Layout-switching in drupal zen subtheme

I just did somethign wrong when creating my own drupal theme by makin a subtheme of the zen theme from the STARTERKIT:

I copied the file layout-fixed as it is without changing it's name to layout.css - which seems to be what you have to do, and what the nice helper script does.

But I want to be able to change between the fixed and the liquid setting in the admin menu.
How can I do that?

I add this to theme-settings.php:

$form['themedev']['zen_layout'] = array(
'#type' => 'radios',
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