macOS NSTabViewController centered tab icons

I have a macOS app I have created. And I have a Preferences window built using a swift & storyboard with a tabless Toolbar styled NSTabViewController.

I see no way to override the positioning of the tab icons though - they default to the left side of the window. Is it possible?

Subclass NSTabViewController and override func toolbarDefaultItemIdentifiers(_ toolbar: NSToolbar) -> [NSToolbarItem.Identifier]. Add a flexibleSpace toolbar item at both sides.

following Willeke's tip, I came up with this override in my NSTabViewController subclass:

override func toolbarDefaultItemIdentifiers(_ toolbar: NSToolbar) -> [NSToolbarItem.Identifier] {

    super.toolbarDefaultItemIdentifiers(toolbar)

    var arr = Array<NSToolbarItem.Identifier>()
    for item in self.tabViewItems {
        if let identifier = item.identifier {
            arr.append(NSToolbarItem.Identifier.init(identifier as! String))
        }
    }

    //insert flexible spaces at first and last index
    arr.insert(NSToolbarItem.Identifier.flexibleSpace, at: 0)
    arr.append(NSToolbarItem.Identifier.flexibleSpace)

    return arr
}