I've posted a short tutorial that demonstrates how to modify the CSS theme for the timetable tutorial:
http://code.daypilot.org/39902/asp-net-timetable-with-custom-css-theme
Instead of the _matrix classes, the Calendar control uses borders on the *_inner elements.
Every cell uses an outer and inner div.
- The outer div has fixed dimensions and position set by the calendar.
- The inner div has no inner styles applied.
The themes apply position:absolute and left, right, top, and bottom CSS styles to the inner div to stretch it to the parent size. This way you don't need to know the parent dimensions (so the theme is universal) and you can add borders of any width (the increased with doesn't change the outer div dimensions).