The Mendix calendar is (as far as I know) not included in the Bootstrap themes. I have custom styles for the .dijitCalendar, which is the containing class for the calendar.
If you are using the bootswatch themes they should already have the styling added for the calendar. Which theme are you using and which version?
If you want to edit the calendar in the Mendix bootswatch version then you can look into the bootstrap.css file on this line: https://github.com/mendix/MxBootswatch/blob/master/theme-builder/flatly/ui/theme-flatly/css/bootstrap.css#L7345
If you want to download the themes individually then you can do so from here:
The themes should work with 5.12 and above.