1.Goto ubuntu 14.04 software center 
2.click for Navigation menubutton on taskbar 
3.The menubutton do not look like it is pressed 
4.Goto user/share/software center
5.open UI folder
6.open widgets folder 
7.open menubutton.py file
8.line 37 ,arrow button are written with the shadowtype OUT
9.        label:In
        
arrow = Gtk.Arrow.new(Gtk.ArrowType.DOWN, Gtk.ShadowType.IN)

        box.pack_start(arrow, False, False, 1)