The design is controlled by CSS styles included in your CSS stylesheet for the given site. The following figures show the messaging web parts' CSS classes:

 

My messages:

 

 

Inbox:

 

 

Outbox:

 

 

Contact list:

 

 

Ignore list:

 

 

Send message:

 

 

View message:

 

Page url: http://devnet.kentico.com/docs/5_0/devguide/index.html?messaging_design_and_styles.htm