ExtJS4 out of the box window styling

app.js

Ext.application({
    name: 'My App',
    launch: function() {
        
        Ext.create('Ext.window.Window',{
            title: 'Custom Window',
            width  : 200,
            height : 150,
             modal: true,
             layout: 'fit',
             items: {
                xtype: 'form',
                html: 'Form Content',
                border: 0,
                buttons: [{
                   text: 'close'
               }]
            }
        }).show();
        
    }
});