From MozillaZine Knowledge Base

Preview: Image:Sunbird-0.9-dayview-with-events-rounded.png

/* CalendarEventBoxRounded.css
 * 
 * To give your event boxes rounded corners,
 * save this css in your profile [1] chrome directory,
 * and import this CSS in your userChrome.css [2]. (References at end.)
 *
 * For Sunbird, the userChrome.css path may be something like the following
 * w2k: %APPDATA%\Mozilla\Sunbird\Profiles\NNNNN.default\chrome\userChrome.css
 * gnu/linux: ~/.mozilla/sunbird/NNNNNN.default/chrome/userChrome.css
 * 
 * 1. Save this file in your profile as chrome/CalendarEventBoxRounded.css
 * 2. Add the following to profile file chrome/userChrome.css (create if none):
 *   @import url("CalendarEventBoxRounded.css");
 *
 * Tested on Sunbird 0.9.
*/

/* Day View, Week View (calendar-event-box used for selected shadow) */
calendar-event-box,
calendar-event-box .calendar-event-selection,
calendar-event-box .calendar-color-box { 
  -moz-border-radius: 1em;
}
calendar-event-box .category-color-box {
  -moz-border-radius: 0em 1em 1em 0em;
}

/* Month View, Multiweek View, Day View All Day, Week View All Day */
calendar-month-day-box-item .calendar-color-box, 
calendar-header-container .calendar-color-box { 
  -moz-border-radius: 0.4em;
}
calendar-month-day-box-item .calendar-event-selection,
calendar-header-container .calendar-event-selection {
  -moz-border-radius: 0.4em 0em 0em 0.4em;
}
calendar-month-day-box-item .category-color-box,
calendar-header-container .category-color-box {
  -moz-border-radius: 0em 0.4em 0.4em 0em;
}

/* References:
 * [1] Where is my profile folder?
 *     (http://kb.mozillazine.org/Profile_folder#Where_is_my_profile_folder.3F)
 * [2] Customizing Mozilla: User CSS
 *     (http://www.mozilla.org/unix/customizing.html)
 * [3] CSS Reference / Mozilla Extensions: -moz-border-radius
 *     (https://developer.mozilla.org/En/CSS/-moz-border-radius)
 * This file is at http://kb.mozillazine.org/Event_box_rounding_CSS_-_Sunbird
 */