1. Under themes, duplicate the skeleton theme and give a new theme name to the new directory created.
2. In view/calendar.css, replace all style name with .yourtheme.
3. In view/longdesc.css, replace all style name with .yourtheme_longdesc.
4. Change the class name in both model/Calendar.php and model/Longdesc.php to QCalendarYourtheme and LongdescYourtheme.
4. Now the big headache, add in the logic for the model (model/Calendar.php, model/Longdesc.php) and view (view/calendar.phtml, view/longdesc.phtml).
5. Have fun!