更改Fullcalendar themeSystem的默认设置

Question

当前使用Bootstrap 4和Fullcalendar.io的新集成。效果惊人，但是我一直在尝试找出如何更改https://github.com/fullcalendar/fullcalendar/blob/master/src/theme/Bootstrap4Theme.ts第14行中定义的按钮类的默认值：

export default class Bootstrap4Theme extends Theme {
}

Bootstrap4Theme.prototype.classes = {
  widget: 'fc-bootstrap4',

  tableGrid: 'table-bordered', // avoid `table` class b/c don't want margins. only border color
  tableList: 'table', // `table` class creates bottom margin but who cares
  tableListHeading: 'table-active',

  buttonGroup: 'btn-group',
  button: 'btn btn-primary',
  stateActive: 'active',
  stateDisabled: 'disabled',

  today: 'alert alert-info', // the plain `info` class requires `.table`, too much to ask
  popover: 'card card-primary',
  popoverHeader: 'card-header',
  popoverContent: 'card-body',

  // day grid
  // for left/right border color when border is inset from edges (all-day in agenda view)
  // avoid `table` class b/c don't want margins/padding/structure. only border color.
  headerRow: 'table-bordered',
  dayRow: 'table-bordered',

  // list view
  listView: 'card card-primary'
}

我想将 .btn-primary 的默认设置修改为 .btn-outline-secondary 。

有什么办法可以通过修改Fullcalendar上Bootstrap 4主题的默认设置来做到这一点？