middle-header region does not collapse even though empty

Hi,
You are right , there is no such call in 'acquia_slate.info' to activate local.css. But I think there is no need to add such code because local.css gets activated when we rename local_sample.css to local.css , by the following code which is there in acquia_slate's 'page.tpl.php'.

  <?php if ($local_styles): ?>
  <?php print $local_styles; ?>
  <?php endif; ?> 

Variable $local_styles has been defined in fusion_core's 'template.php' by the following code :

  $vars['local_styles'] .= (file_exists($path . '/css/local.css')) ? $link . '/css/local.css" />' . "\n" : '';

Rgrds,

Deepika Chavan.

Or you can add the "local.css" file to load through your theme by adding this line to your theme's .info file.

stylesheets[all][] = local.css

Hope that helps!