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You can change the thank you page font easily.
All you have to do is to click on Thank You link and then choose Thank You message.
After clicking on Next button you can see a window like the below image where you can change the Font family.
I believe the easiest way to achieve this is by using a custom URL on the that you page:
This way you will be using an external html file and having the freedom to use your css files as you wish.
Else, I did some tests using the "thank you message" option which allows to use the HTML editor:
The idea was to use inline css which in theory is possible but I could not make it work. You can see more details here.
Let us know if this is a solution for you.