Increase font size of response message

Since we can’t put html into a message, is there a way to increase the size of the auto-response when someone has filled in a form? The current response is small type inside a green box. Some people (okay, two people so far) didn’t notice it and didn’t think his message went through.

I’d like it to be in text that’s much larger than the rest of the page, or in a big box with lots of white space, so it’s noticeable.

Thanks!

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