Add dark/light mode toggling tests

tests/test_dark_toggle.py

@@ -0,0 +1,38 @@
+from textual.app import App
+
+class OnLoadDarkSwitch(App[None]):
+    """App for testing toggling dark mode in on_load."""
+
+    def on_load(self) -> None:
+        self.dark = not self.dark
+
+async def test_toggle_dark_on_load() -> None:
+    """It should be possible to toggle dark mode in on_load."""
+    async with OnLoadDarkSwitch().run_test() as pilot:
+        assert not pilot.app.dark
+
+class OnMountDarkSwitch(App[None]):
+    """App for testing toggling dark mode in on_mount."""
+
+    def on_mount(self) -> None:
+        self.dark = not self.dark
+
+async def test_toggle_dark_on_mount() -> None:
+    """It should be possible to toggle dark mode in on_mount."""
+    async with OnMountDarkSwitch().run_test() as pilot:
+        assert not pilot.app.dark
+
+class ActionDarkSwitch(App[None]):
+    """App for testing toggling dark mode from an action."""
+
+    BINDINGS = [("d", "toggle", "Toggle Dark Mode")]
+
+    def action_toggle(self) -> None:
+        self.dark = not self.dark
+
+async def test_toggle_dark_in_action() -> None:
+    """It should be possible to toggle dark mode with an action."""
+    async with OnMountDarkSwitch().run_test() as pilot:
+        await pilot.press("d")
+        await pilot.pause(2 / 100)
+        assert not pilot.app.dark