This module is an attempt to make it easier to theme the messages generated by Drupal's drupal_set_message() function. I've seen many well crafted Drupal themes where the themer overlooked the status messages - often just because they are fleeting and inconvenient to generate. This can lead to usability issues when users fail to notice messages.
The Message Themer module generates these messages on every page so that they are always available for styling. It also allows you to choose which types of messages to display (status, warning, or error types) and how many messages to display in each type.
