test fix

src/textual/app.py

@@ -368,6 +368,7 @@ class App(Generic[ReturnType], DOMNode):
         self._screenshot: str | None = None
         self._dom_lock = asyncio.Lock()
         self._dom_ready = False
+        self.set_class(self.dark, "-dark-mode")
 
     @property
     def return_value(self) -> ReturnType | None:

src/textual/widgets/_header.py

@@ -91,13 +91,14 @@ class Header(Widget):
     Header {
         dock: top;
         width: 100%;
-        background: $secondary-background;
+        background: $foreground 5%;
         color: $text;
         height: 1;
     }
     Header.-tall {
         height: 3;
     }
+    
     """
 
     tall = Reactive(False)

tests/test_test_runner.py

@@ -12,7 +12,9 @@ async def test_run_test() -> None:
 
     app = TestApp()
     async with app.run_test() as pilot:
-        assert str(pilot) == "<Pilot app=TestApp(title='TestApp')>"
+        assert (
+            str(pilot) == "<Pilot app=TestApp(title='TestApp', classes={'-dark-mode'})>"
+        )
         await pilot.press("tab", *"foo")
         await pilot.pause(1 / 100)
         await pilot.exit("bar")