Border around panel

I have Viewport with center region and classic theme style:

Code:

{
            region: 'center',
            bodyBorder: false,
            border:false,
            xtype: 'mainpanel',
            reference: 'mainPanel'
        }

and mainPanel:

Code:

Ext.define('myApp.view.main.MainPanel', {
    extend: 'Ext.tab.Panel',
    alias: 'widget.mainpanel',
    border: false,
    bodyBorder: false,
    ...

But there is border around main panel. Why?

If I change theme to Neptune than the border is gone. But I would like to have classic theme.

Any idea how to remove the border?

Border around panel