These docs are for an old release. Info on upgrading to v6
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themeSystem

Renders the calendar with a given theme system.

String, default: 'standard'

'standard'
Renders a minimal look & feel, the look in most of the demos. Does not require any CSS files beyond the FullCalendar base files.
'bootstrap5'
Prepares the calendar for a Bootstrap 5 theme. Further instructions »
'bootstrap'
Prepares the calendar for a Bootstrap 4 theme. Further instructions »