user-accounts: Make the UmCarouselItems look the same on hover

UmCarouselItem is a button, but since we have now the arrow which
points to the selected item, we don't need any other visual
feedback (such as hover border/background).

https://bugzilla.gnome.org/show_bug.cgi?id=767065

panels/user-accounts/data/carousel.css

@@ -18,3 +18,10 @@
   border-bottom-color: @theme_bg_color;
   margin-bottom: -2px;
 }
+
+.carousel-item {
+  background: transparent;
+  box-shadow: none;
+  border: none;
+  color: @theme_fg_color;
+}

panels/user-accounts/um-carousel.c

@@ -48,6 +48,8 @@ static void
 um_carousel_item_init (UmCarouselItem *self)
 {
         gtk_toggle_button_set_mode (GTK_TOGGLE_BUTTON (self), FALSE);
+        gtk_style_context_add_class (gtk_widget_get_style_context (GTK_WIDGET (self)),
+                                     "carousel-item");
 }
 
 struct _UmCarousel {