Allow App.dark to be set before there's a screen

This commit addresses Textualize#1369 by swallowing any `ScreenStackError` when
calling `refresh_css` after setting the -{dark,light}-mode class on the app.
The idea here being that if there is no screen, that's fine, the style will
be taken up once there is one.

src/textual/app.py

@@ -480,7 +480,14 @@ def watch_dark(self, dark: bool) -> None:
         """Watches the dark bool."""
         self.set_class(dark, "-dark-mode")
         self.set_class(not dark, "-light-mode")
-        self.refresh_css()
+        try:
+            self.refresh_css()
+        except ScreenStackError:
+            # It's possible that `dark` can be set before we have a default
+            # screen, in an app's `on_load`, for example. So let's eat the
+            # ScreenStackError -- the above styles will be handled once the
+            # screen is spun up anyway.
+            pass
 
     def get_driver_class(self) -> Type[Driver]:
         """Get a driver class for this platform.