feat(client_utils): theme can be changed from the CLI

bec_widgets/cli/client_utils.py

@@ -297,6 +297,25 @@ class BECGuiClient(RPCBase):
             return self._raise_all()
         return self._start(wait=wait)
 
+    def change_theme(self, theme: Literal["light", "dark"] | None = None) -> None:
+        """
+        Apply a GUI theme or toggle between dark and light.
+
+        Args:
+            theme(Literal["light", "dark"] | None): Theme to apply. If None, the current
+                theme is fetched from the GUI and toggled.
+        """
+        if not self._check_if_server_is_alive():
+            self._start(wait=True)
+
+        with wait_for_server(self):
+            if theme is None:
+                current_theme = self.launcher._run_rpc("fetch_theme")
+                next_theme = "light" if current_theme == "dark" else "dark"
+            else:
+                next_theme = theme
+            self.launcher._run_rpc("change_theme", theme=next_theme)
+
     def new(
         self,
         name: str | None = None,

bec_widgets/widgets/containers/main_window/main_window.py

@@ -254,7 +254,7 @@ class BECMainWindow(BECWidget, QMainWindow):
         self.ui = loader.loader(ui_file)
         self.setCentralWidget(self.ui)
 
-    def _fetch_theme(self) -> str:
+    def fetch_theme(self) -> str:
         return self.app.theme.theme
 
     def _get_launcher_from_qapp(self):

tests/unit_tests/test_client_utils.py

@@ -86,3 +86,44 @@ def test_client_utils_passes_client_config_to_server(bec_dispatcher):
                 config=mixin._client._service_config.config,
                 logger=mock.ANY,
             )
+
+
+@contextmanager
+def _no_wait_for_server(_client):
+    yield
+
+
+@pytest.mark.parametrize("theme", ["light", "dark"])
+def test_client_utils_apply_theme_explicit(theme):
+    gui = BECGuiClient()
+    launcher = mock.MagicMock()
+
+    with mock.patch.object(
+        BECGuiClient, "launcher", new_callable=mock.PropertyMock
+    ) as launcher_prop:
+        launcher_prop.return_value = launcher
+        with mock.patch("bec_widgets.cli.client_utils.wait_for_server", _no_wait_for_server):
+            with mock.patch.object(gui, "_check_if_server_is_alive", return_value=True):
+                gui.change_theme(theme)
+
+    launcher._run_rpc.assert_called_once_with("change_theme", theme=theme)
+
+
+@pytest.mark.parametrize("current_theme, expected_theme", [("light", "dark"), ("dark", "light")])
+def test_client_utils_apply_theme_toggles_when_none(current_theme, expected_theme):
+    gui = BECGuiClient()
+    launcher = mock.MagicMock()
+    launcher._run_rpc.side_effect = [current_theme, None]
+
+    with mock.patch.object(
+        BECGuiClient, "launcher", new_callable=mock.PropertyMock
+    ) as launcher_prop:
+        launcher_prop.return_value = launcher
+        with mock.patch("bec_widgets.cli.client_utils.wait_for_server", _no_wait_for_server):
+            with mock.patch.object(gui, "_check_if_server_is_alive", return_value=True):
+                gui.change_theme(None)
+
+    assert launcher._run_rpc.call_args_list == [
+        mock.call("fetch_theme"),
+        mock.call("change_theme", theme=expected_theme),
+    ]