The problem is that whoever wrote your calendar (plugin or part of the theme?) decided to add the following CSS rule with the !important directive.
.calendar-table table, tbody, tr, td {
margin: 0 !important;
padding: 0 !important;
I cannot tell you how much it annoys me when a developer uses this directive on a basic tag like tr or td. It destroys the entire stylesheet cascade (the “C” in CSS). Bad, bad, bad, bad, bad!!! (Let’s give them the benefit of the doubt, and assume they just don’t know any better.)
I might be able to fix this in your table, but I’m not sure if I can or if I should. The right answer is for you to contact the developer of the calendar and ask them to REMOVE THE !IMPORTANT from anything that is not tagged as a ‘calendar element’.
-Mark