When the user submits the contact form on the Divi contact form they get the 'thank you message', however to change the font and justification of this message, add the below custom CSS and edit to suit:

#page-container .et_pb_contact_form_0.et_pb_contact_form_container .et-pb-contact-message {
color:aquamarine;
text-align: center;
border: 1px solid lightgreen;
background: orange;
}