How to use the fullcalendar.defineThemeSystem function in fullcalendar

// noinspection ES6UnusedImports
import style from '../css/calendar.scss';
import moment from "moment";

import * as fullcalendar from "fullcalendar";
import {FullCalendarMSTTheme} from "./FullCalendarMSTTheme";

require("fullcalendar/dist/locale/de");

fullcalendar.defineThemeSystem('mst', FullCalendarMSTTheme);

$(function () {
    let $calendar = $('#calendar'),
        language = $('html').attr('lang'),
        defaultView = $calendar.data('default-view'),
        defaultDate = moment($calendar.data('default-date'), "YYYY-MM-DD"),
        initView = $calendar.data('init-view'),
        initDate = $calendar.data('init-date'),
        dataSrc = $calendar.data('src');

    $calendar.fullCalendar({
        header: {
            left: 'prev,next today',
            center: 'title',
            right: 'month,agendaWeek,agendaDay listYear'
        },