Hi,
The CSS file you are looking for is probably located at "
CMSAdminControls\ModalCalendar\Themes\Default\jquery-ui-1.8.7.custom.css". This is the default stylesheet for date picker and you can change it as you like.
Since jQuery UI is used for date picker, you can use
http://jqueryui.com/themeroller/ to easy generate theme to fit your page. There is a little drawback with this approach. This themeroller is newer than the one used in Kentico and has some different fields that might not be fully compatible with the old one and the classes are also different. That means that when you generate your theme using themeroller you would need to change classes to fit the old ones that kentico uses.
Best regards,
Richard Sustek