Rebrand as Playpen.

src/View.zig

@@ -93,7 +93,7 @@ pub fn create(alloc: std.mem.Allocator, cbs: Callbacks) !*View {
         .ctx = cbs.ctx,
     };
 
-    self.box.as(gtk.Widget).addCssClass("vtabs-view");
+    self.box.as(gtk.Widget).addCssClass("playpen-view");
     self.box.as(gtk.Widget).setHexpand(1);
     self.box.as(gtk.Widget).setVexpand(1);