Attaching css to blocks
A block array has a 'subject' and 'content' key, it's just a convention. See hook_block_view() for details:


  Return value
  
  Either an empty array so the block will not be shown or an array containing the following elements:
  
  
  subject: The default localized title of the block. If the block does not have a default title, this should be set to NULL.
  content: The content of the block's body. This may be a renderable array (preferable) or a string containing rendered HTML content. If the content is empty the block will not be shown.