calendar widget

Hi, Please forgive my ignorance if I’m way off here. I am using the ( I’m guessing ) default calendar widget (not a plugin but one that appears in inactive widget area ) but I don’t know if that’s specific to Responsive theme or just wordpress. There is a very beautiful calendar here: https://www.jefvlamings.com/blog/a-clean-calendar-in-css3-jquery/ that I would like to try to rework the css on this widget to create but I don’t know where the code for the calendar is to see if I can modify it to get something close visually. It seems a perfect fit for this lovely and minimalistic looking theme. Thanks for sharing this theme and for the time that you put into reading and addressing these queries!!