Yes, even though, unlike other content displayed in advanced integration, the calendar previews are embedded in an iframe, you can still customize the look&feel of them.
At this page you can set the URL of a custom stylesheet file. You should place this file on your server. It will be used every time a calendar preview is displayed for your checkin24-7 site. Please note that the stylesheet doesn't need to include all styles, simply add the styles needed to accomplish the desired look&feel. Your styles will override the styles used by checkin24-7 by default.