How to change background colour of ThemedGenBitmapTextButton?

I create a ThemedGenBitmapTextButton object. I try to change its background colour using SetBackgroundColour() method, but it doesn’t work. Could you please help?

from wx.lib.buttons import ThemedGenBitmapTextButton

self.new_button = ThemedGenBitmapTextButton(self, -1, self.new_button_bmp, label=“Test Button”, pos=(-1,-1), size=(320,128))

self.new_button.SetBackgroundColour(wx.Colour(188,188,188))