Bootstrap 4 Theming

In order to correctly theme your calendar with a Bootstrap 4 theme, you must include the correct stylesheets, include the JavaScript plugin, and set themeSystem to 'bootstrap'.

Using a Bundle

You can use the Bootstrap plugin with browser globals and script tags. First, the Bootstrap stylesheet must be separately loaded in its own <link> tag. Also, a FontAwesome stylesheet must be loaded (more info). Example:

<link href='https://cdn.jsdelivr.net/npm/bootstrap@4.5.0/dist/css/bootstrap.css' rel='stylesheet'>
<link href='https://cdn.jsdelivr.net/npm/@fortawesome/fontawesome-free@5.13.1/css/all.css' rel='stylesheet'>

Then, load a FullCalendar bundle and initialize the calendar:

<link href='fullcalendar/main.css' rel='stylesheet' />
<script src='fullcalendar/main.js'></script>
<script>
...
var calendar = new FullCalendar.Calendar(calendarEl, {
  themeSystem: 'bootstrap'
});
...
</script>

Using an ES6 Build System

Alternatively, you can load the bootstrap plugin using an ES6 build system. Install the necessary FullCalendar packages first:

npm install --save @fullcalendar/core @fullcalendar/bootstrap

Then, install the packages for Bootstrap and FontAwesome:

npm install --save bootstrap@4 @fortawesome/fontawesome-free

Then, import the necessary packages and initialize your calendar:

// import the third-party stylesheets directly from your JS
import 'bootstrap/dist/css/bootstrap.css';
import '@fortawesome/fontawesome-free/css/all.css'; // needs additional webpack config!

import { Calendar } from '@fullcalendar/core';
import bootstrapPlugin from '@fullcalendar/bootstrap';
...
var calendar = new Calendar(calendarEl, {
  plugins: [ bootstrapPlugin ],
  themeSystem: 'bootstrap'
});
...

Using fontawesome-free with webpack is a bit complicated because it relies on font files that are not present in the main stylesheet. See an example project that does it

3rd Party Bootstrap Themes

You can use the standard Bootstrap theme downloaded from getbootstrap.com.

You can also use themes downloaded from 3rd party providers. Google “free bootstrap themes” as a starting point.

The theme you download might already have custom CSS written to style FullCalendar. If so, all the above steps of including the plugin files and specifying theme: 'bootstrap' are unnecessary. If in doubt, try both methods to see which one looks better.