[Plugin: Contact Form 7] Customizing design of error messages

Hi, thanks for that great plugin! I really love using it but now my boss asked me to change the look of the error mesages. So, how can I do that in general?

And: is there a way to change the behavior of the notification at all? e.g. I would like to get a red border around the input fields instead the textbox with the message over the input field. Possible?

Thanks a lot in advance!

https://www.ads-software.com/extend/plugins/contact-form-7/