This can be done with HTML. In the invite editor, click the "Source" icon to add the code for the header and background color.

I'm assuming you are talking about the email message?
It may be helpful to know that your email message is customizable separately to the survey itself.
When writing the email invitation, there is a bunch of icons above where you write the text. It has things like font, size, inclusion of pictures/banners, etc. This is where you go to customize:

One thing you can't easily do change color.
For extra customization, you need to apply html code.
Hope this helps.
Cheers