If you have light and dark sets of your icon,you can make it work by writing these codes on your Loaded event of PhoneApplication Page:
|  private void PhoneApplicationPage_Loaded(object sender, RoutedEventArgs e)
        {
            var visibility = (Visibility)Application.Current.Resources["PhoneLightThemeVisibility"];
            if (visibility == Visibility.Visible)
            {
                 
                var button = (IApplicationBarIconButton)ApplicationBar.Buttons[1];
                button.IconUri=new Uri("H_light.png",UriKind.Relative);
            }
 
            else
            {
                var button = (IApplicationBarIconButton)ApplicationBar.Buttons[1];
                button.IconUri = new Uri("H_dark.png", UriKind.Relative);
            }
        }
 | 
So after building and running the project,it'll be working like a charm.
Hope it helps!