Hi Rigel,
The calendar, widgets, etc. can work with a responsive theme. But please keep in mind that users do sometimes encounter theme-specific problems. You’ll probably have no trouble, but I just wanted to let you know that we can only provide limited support for some of the obscure and highly customized themes out there.
I recommend that you try using our free plugin on your site first, and see how it looks. Adding PRO will give you added functionality, but testing with the free version will give you a good idea of how things will look. Also, check out our Themers Guide at https://theeventscalendar.com/support/documentation/events-calendar-themers-guide/ If your theme uses Genesis, you should also read through https://theeventscalendar.com/tutorial-integrating-the-events-calendar-w-genesis/
I hope helps! If you have anymore questions, don’t hesitate to ask.
Cheers,
Leah