[Plugin: Contact Form 7] Style the "Please fill the required field" without popping up

Hi,

I’m relay enjoying your plugin and also use it for all my company pages!

But I have one question. I would like to change the style of the message that pops up when you forget filling out a field. Currently it just shows the standard red popups that don’t fit to the design of my page.

Here is the form I am concerned of: http://shadesofclass.com/corporate/contribute/

This is what I want to do (Just press submit without filling smth. out):
http://press.designhotels.com/press/register

Would be wonderful if you could help me!!

Regards Antoine

http://wordpress.org/extend/plugins/contact-form-7/