How to overwrite the add content page
Maybe implement hook_theme() in your template.php and add an extra template for the test_node_form:

return array('test_node_form' =&gt; 
    array('arguments' =&gt; 
        array('form' =&gt; NULL), 'template' =&gt; 'test-node-edit'
    )
);


Now you can write a test-node-edit.tpl.php for the form. But that's only for the form, not the whole page.