fullCalendar and bootstrap theme

Question

I try to use the "bootstrap5" theme on the excellent fullCalendar 6.

I followed the documentation.

My config is :

const calendar = new FullCalendar.Calendar(calendarEl, {
                initialView: 'timeGridDay',
                locale: 'fr',
                themeSystem: 'bootstrap5',
                headerToolbar: {
                    left: 'prev,next today',
                    center: 'title',
                    right: 'dayGridMonth,timeGridWeek,timeGridDay,listMonth'
                },

The page source looks like that (this picture to show that everything is present)

But the displayed theme is the standard one :

What can be my error ?