Hey there! Thank you for submitting to our Feature requests!
You can always change the font as well as the color of the "Thank you" message text or background. Simply click on the Form element > Edit Styles and you can make the needed changes in Fonts and Colors sections respectively. However, we'll make sure to pass your request to our tech team, so that they can find the best solution for it.
If you have any other questions, feel free to contact us via our Live Chat.