These docs are for an old release. Info on upgrading to v4
edit doc

themeButtonIcons

Determines which icons are displayed in buttons of the header/footer when jQuery UI theming is on.

Object, default:

{
  prev: 'circle-triangle-w',
  next: 'circle-triangle-e',
  prevYear: 'seek-prev',
  nextYear: 'seek-next'
}

This option only applies to calendars that have themeSystem set to 'jquery-ui'.

A hash must be supplied that maps button names (from the header) to icon strings. The icon strings determine the CSS class that will be used on the button. For example, the string 'circle-triangle-w' will result in the class 'ui-icon-circle-triangle-w'.

If a button does not have an entry, it falls back to using buttonText.

If you are using a jQuery UI theme and would prefer not to display any icons and would rather use buttonText instead, you can set the themeButtonIcons option to false.