Toolbar button style

I create toolbar with gtkuimanager in my gtk+ application. How can i change style o开发者_Python百科f toolbar button?

Thank you.

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    #   set a vertical toolbar to the left
    self.tools   = gtk.Toolbar()
    toolbar_item = gtk.ToolButton()
    toolbar_item.set_stock_id(gtk.STOCK_APPLY)
    toolbar_item.set_label("Show Levels")
    toolbar_item.show()
    self.tools.insert(toolbar_item, -1)
    self.tools.set_orientation(gtk.ORIENTATION_VERTICAL)
    containerH.pack_start(self.tools, False, False, 0)