Why the confirm mail not use the CSS text plain?

Hi Ish,
i have activated the Mail and Mail2 in Contact Form.
I’d like to apply a graphic layout to those mail.
Perhaps the plugin is corrupt?
Do you know anything about?

https://www.dropbox.com/s/t1fi83lgyc0cobn/Schermata%202015-04-27%20alle%2009.17.59.png?dl=0

Thx for your help!
Davide