How do I use '#attached' to attach a style sheet?
You cannot use #attached inside your hook_theme implementation. Instead, you should add it when you're building a render array:

$blocks['content'] = $render;
$blocks['content']['#attached'] = array (
  'css' =&gt; array(
    drupal_get_path('module', 'mymodule') . '/mymodule.css',
  ),
);


As kiamlaluno already pointed out in the comments, in your code example $blocks['content'] is the render array, so the #attached element must be part of that array.