Hi Kari
The file plugins/contact-form-7/includes/css/styles.css is the file that creates the notification styles used in Contact Form 7. I am able to make changes to the notification styles by making changes in this file.
I wouldn't however make permanent changes here. The more common method is to create a child theme & make any necessary changes in the child themes style.css. That way you don't lose your changes when you update the plugin.
If your changes are not coming through they may be being overwritten by something else in your installation - perhaps your theme or another plugin. You could use Firebug to examine what is happening in the css heirachy.