Change Color Scheme of Thank You Message

I've just set up a way for people to donate via Paypal and have tested to see if it works. It does work but upon completing the transaction the recipient is greeted with a 'Thank You' message on my site but the colour theme is incorrect and I would like to sort that. The content of the Thank You message is fine, but I am at a loss as to where to find the option to either change the content of the Thank You message or even simply adjust the Colour theme.

TIA.