Murrine

Now I can confirm it's a bug. One workaround is to set bg color of notebook to the same as normal bg color, which the official Human theme does. And I got this comment as well:  style "murrine-spinbutton"
{
        #bg[NORMAL]      = shade (0.95, @bg_color) # we need to fix rectangular border bug on spinbutton
        bg[INSENSITIVE] = shade (0.99, @bg_color)         engine "murrine"
        {
                gradient_shades = {1.02,1.00,0.99,0.95}
        }
}