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.