Thank you message in the iFrame
On the form, go to Settings tab
Scroll down and make sure that the "Show Thank You Page within iFrame" and "Use Custom Thank You page" are ticked.
Click the Edit link to edit the thank you message
Update the CSS to change the style of the font and update the text inside <h1>...</h1> and/or <p>...</p> to change the message.
Click Save Custom Thank You page.