how to set default class for buttons ?

Hi,
I use my own color scheme so I need to css the datatable also. Since I use multiple instances of datatables, I want to set it for all in one js file.
Therefore I created a [name]-defaults.js with the code:

$.extend( $.fn.dataTable.defaults, {
responsive: true,
// other settings...
language: {
....
language settings
......
}
} );

but could not find a way to use buttons className: 'my-customs-class'

Sugestions?