[Light Switch] Enable theme switching with schedule by Jaylyn-Barbee · Pull Request #44200 · microsoft/PowerToys

Jaylyn-Barbee · 2025-12-10T16:41:25Z

Summary of the Pull Request

This PR introduces behavior to allow users to attach a theme file to both light and dark mode which will switch when the color mode changes.

PR Checklist

Closes: Select specific light/dark themes #42413
Communication: I've discussed this with core contributors already. If the work hasn't been agreed, this work might be rejected
Localization: All end-user-facing strings can be localized
Documentation updated: If checked, please file a pull request on our docs repo and link it here: #xxx

Notes

How do we tell people that their theme files are saved at C:\Users\username\AppData\Local\Microsoft\Windows\Themes, the file picker just opens to a default location.
Double check that the theme is applying both on schedule and with the use of the shortcut.
On back burner for now until we can figure out a better solution for the Settings app issue.

karlingen · 2025-12-27T15:33:05Z

Thank you for your work on this! What's left to get this PR published?

Jaylyn-Barbee · 2026-01-05T15:18:57Z

@karlingen

As it stands, the code in this PR will invoke the theme file just with the side effect of opening the settings app.
I am struggling to find a solution in C++ to invoke this file without the settings app opening. This is the only thing blocking this PR besides polishing things up and testing.

Copilot

Pull request overview

Copilot reviewed 27 out of 27 changed files in this pull request and generated 12 comments.

Copilot · 2026-01-12T14:05:24Z

+// Aggressively suppress Settings window - hide, move off-screen, close, and terminate
+static void SuppressSettingsWindow(HWND hwnd)
+{
+    // First, immediately hide it to prevent flash
+    ShowWindow(hwnd, SW_HIDE);
+
+    // Move it off-screen as backup
+    SetWindowPos(hwnd, nullptr, -32000, -32000, 0, 0, 
+        SWP_NOSIZE | SWP_NOZORDER | SWP_NOACTIVATE);
+
+    // Send close message
+    PostMessageW(hwnd, WM_CLOSE, 0, 0);
+
+    // Also terminate the process for faster closure
+    DWORD processId = 0;
+    GetWindowThreadProcessId(hwnd, &processId);
+    if (processId != 0)
+    {
+        HANDLE hProcess = OpenProcess(PROCESS_TERMINATE, FALSE, processId);
+        if (hProcess)
+        {
+            TerminateProcess(hProcess, 0);
+            CloseHandle(hProcess);
+        }
+    }


Aggressive process termination: Using TerminateProcess forcefully kills SystemSettings.exe without graceful shutdown. This could potentially lead to data loss or corruption in Settings app state. Consider if this aggressive approach is necessary, or if PostMessage(WM_CLOSE) alone would suffice. Document why forceful termination is required if this behavior is intentional.

Suggested change

// Aggressively suppress Settings window - hide, move off-screen, close, and terminate

static void SuppressSettingsWindow(HWND hwnd)

{

// First, immediately hide it to prevent flash

ShowWindow(hwnd, SW_HIDE);

// Move it off-screen as backup

SetWindowPos(hwnd, nullptr, -32000, -32000, 0, 0,

SWP_NOSIZE | SWP_NOZORDER | SWP_NOACTIVATE);

// Send close message

PostMessageW(hwnd, WM_CLOSE, 0, 0);

// Also terminate the process for faster closure

DWORD processId = 0;

GetWindowThreadProcessId(hwnd, &processId);

if (processId != 0)

{

HANDLE hProcess = OpenProcess(PROCESS_TERMINATE, FALSE, processId);

if (hProcess)

{

TerminateProcess(hProcess, 0);

CloseHandle(hProcess);

}

}

// Suppress Settings window - hide, move off-screen, and request graceful close

static void SuppressSettingsWindow(HWND hwnd)

{

// First, immediately hide it to prevent flash

ShowWindow(hwnd, SW_HIDE);

// Move it off-screen as backup so it's not visible even if shown briefly

SetWindowPos(hwnd, nullptr, -32000, -32000, 0, 0,

SWP_NOSIZE | SWP_NOZORDER | SWP_NOACTIVATE);

// Request a graceful shutdown instead of forcefully terminating the process

PostMessageW(hwnd, WM_CLOSE, 0, 0);

Copilot · 2026-01-12T14:05:25Z

+
+            var picker = new FileOpenPicker();
+
+            var window = new Window();


Creating a new Window instance may cause issues or resource leaks. The picker should typically be initialized with the actual application window handle. Consider obtaining the window handle from the current UI window (e.g., using App.GetMainWindow() or similar pattern) instead of creating a new Window instance.

Suggested change

var window = new Window();

var window = App.GetMainWindow();

Copilot · 2026-01-12T14:05:25Z

              { L"SunsetToSunrise", L"Use sunrise/sunset times" },
-              { L"FollowNightLight", L"Follow Windows Night Light state" }
-            });
+              { L"FollowNightLight", L"Follow Windows Night Light state" } });


Inconsistent whitespace formatting. There's an unnecessary space before the closing braces in the initializer list. This should follow consistent formatting with the rest of the file.

Copilot · 2026-01-12T14:05:25Z

    if (!_state.isManualOverride && (appsNeedsToChange || systemNeedsToChange))
    {
        Logger::info(L"[LightSwitchStateManager] Applying {} theme", shouldBeLight ? L"light" : L"dark");
+


Trailing whitespace after the opening brace. This should be removed to maintain code cleanliness.

Suggested change

Copilot · 2026-01-12T14:05:26Z

+void StopSettingsMonitor()
+{
+    if (!g_monitorRunning)
+    {
+        return;
+    }
+
+    g_stopMonitoring = true;
+
+    if (g_monitorThread.joinable())
+    {
+        g_monitorThread.join();
+    }
+
+    g_monitorRunning = false;


Potential race condition: The g_monitorRunning flag is set to false after joining the thread, but g_stopMonitoring is set to true before the join. If another thread calls StartSettingsMonitor between setting g_stopMonitoring and resetting g_monitorRunning, it could lead to inconsistent state. Consider using a mutex to protect the state transitions or ensure atomic operations.

Copilot · 2026-01-12T14:05:26Z

+                                                x:Uid="LightSwitch_ThemeButtonSelectFolder"
+                                                Grid.Column="1"
+                                                Click="ThemeFilePickerButton_Click"
+                                                Content="{ui:FontIcon Glyph=&#xe8da;,
+                                                                      FontSize=12}"
+                                                Tag="Light">
+                                                <ToolTipService.ToolTip>
+                                                    <ToolTip>
+                                                        <TextBlock x:Uid="LightSwitch_ThemeButtonSelectLocation" />
+                                                    </ToolTip>
+                                                </ToolTipService.ToolTip>
+                                            </Button>
+                                        </Grid>
+                                    </tkcontrols:SettingsCard>
+                                    <tkcontrols:SettingsCard x:Uid="LightSwitch_DarkThemePath" IsEnabled="{x:Bind ViewModel.UseThemeSwitching, Mode=TwoWay}">
+                                        <Grid ColumnSpacing="8">
+                                            <Grid.ColumnDefinitions>
+                                                <ColumnDefinition Width="Auto" />
+                                                <ColumnDefinition Width="Auto" />
+                                            </Grid.ColumnDefinitions>
+                                            <TextBlock
+                                                x:Name="darkPathTextBlock"
+                                                VerticalAlignment="Center"
+                                                Foreground="{ThemeResource TextFillColorSecondaryBrush}"
+                                                IsTextSelectionEnabled="True"
+                                                Style="{StaticResource CaptionTextBlockStyle}"
+                                                Text="{x:Bind ViewModel.DarkThemePath, Mode=TwoWay}"
+                                                TextWrapping="Wrap">
+                                                <ToolTipService.ToolTip>
+                                                    <ToolTip IsEnabled="{Binding IsTextTrimmed, ElementName=darkPathTextBlock, Mode=OneWay}">
+                                                        <TextBlock Text="{x:Bind ViewModel.DarkThemePath, Mode=TwoWay}" TextWrapping="Wrap" />
+                                                    </ToolTip>
+                                                </ToolTipService.ToolTip>
+                                            </TextBlock>
+                                            <Button
+                                                x:Uid="LightSwitch_ThemeButtonSelectFolder"


Missing localization resource: The x:Uid "LightSwitch_ThemeButtonSelectFolder" is used in the XAML but does not have a corresponding entry in the Resources.resw file. This will cause localization errors. Either add the resource or remove the x:Uid attribute if not needed.

Copilot · 2026-01-12T14:05:27Z

+void StopSettingsMonitor()
+{
+    if (!g_monitorRunning)
+    {
+        return;
+    }
+
+    g_stopMonitoring = true;
+
+    if (g_monitorThread.joinable())
+    {
+        g_monitorThread.join();
+    }
+
+    g_monitorRunning = false;


Potential resource leak: If g_monitorThread.joinable() returns false but g_monitorRunning is true, the thread could be running but unjoinable. Consider resetting g_monitorRunning atomically with the thread state check to prevent state inconsistencies.

Copilot · 2026-01-12T14:05:27Z

+                NotifyObservers(SettingId::DarkThemePath);
+            }
+        }
+


Trailing whitespace at the end of line. This should be removed to maintain code cleanliness.

Suggested change

Copilot · 2026-01-12T14:05:27Z

+        public string LightThemePath
+        {
+            get => ModuleSettings.Properties.LightThemePath.Value;
+            set
+            {
+                if (ModuleSettings.Properties.LightThemePath.Value != value)
+                {
+                    ModuleSettings.Properties.LightThemePath.Value = value;
+                    NotifyPropertyChanged();
+                }
+            }
+        }
+
+        public string DarkThemePath
+        {
+            get => ModuleSettings.Properties.DarkThemePath.Value;
+            set
+            {
+                if (ModuleSettings.Properties.DarkThemePath.Value != value)
+                {
+                    ModuleSettings.Properties.DarkThemePath.Value = value;
+                    NotifyPropertyChanged();
+                }
+            }
+        }


Missing validation for theme file paths. Users could potentially set empty or invalid paths through the UI. Consider adding validation in the setter or before saving to ensure paths are non-empty when UseThemeSwitching is enabled.

Copilot · 2026-01-12T14:05:28Z

+        std::this_thread::sleep_for(std::chrono::milliseconds(10));
+    }
+}
+
+// Start the preemptive Settings monitor - call this before any operation that might trigger Settings
+void StartSettingsMonitor()
+{
+    // If already running, do nothing
+    if (g_monitorRunning.exchange(true))
+    {
+        return;
+    }
+
+    g_stopMonitoring = false;
+    g_monitorThread = std::thread(PreemptiveSettingsMonitor);
+
+    // Small delay to ensure monitor is running
+    std::this_thread::sleep_for(std::chrono::milliseconds(50));
+}
+
+// Stop the preemptive Settings monitor
+void StopSettingsMonitor()
+{
+    if (!g_monitorRunning)
+    {
+        return;
+    }
+
+    g_stopMonitoring = true;
+
+    if (g_monitorThread.joinable())
+    {
+        g_monitorThread.join();
+    }
+
+    g_monitorRunning = false;
+
+    // Final cleanup pass
+    for (int i = 0; i < 5; i++)
+    {
+        FindAndSuppressSettings();
+        std::this_thread::sleep_for(std::chrono::milliseconds(100));
+    }
+}
+
+void SetThemeFile(const std::wstring& themeFilePath)
+{
+    // Check if monitor is already running from external call (e.g., ToggleTheme)
+    bool externalMonitor = g_monitorRunning;
+
+    if (!externalMonitor)
+    {
+        // Start our own monitor
+        StartSettingsMonitor();
+    }
+
+    // Execute the theme file
+    SHELLEXECUTEINFOW sei{};
+    sei.cbSize = sizeof(sei);
+    sei.fMask = SEE_MASK_FLAG_NO_UI | SEE_MASK_NOCLOSEPROCESS | SEE_MASK_NOASYNC;
+    sei.lpVerb = L"open";
+    sei.lpFile = themeFilePath.c_str();
+    sei.nShow = SW_HIDE;  // Try to hide from the start
+
+    if (!ShellExecuteExW(&sei))
+    {
+        DWORD err = GetLastError();
+        Logger::error(L"[LightSwitch] ShellExecuteExW failed ({}): {}", err, themeFilePath);
+        if (!externalMonitor)
+        {
+            StopSettingsMonitor();
+        }
+        return;
+    }
+
+    if (sei.hProcess)
+    {
+        WaitForInputIdle(sei.hProcess, 3000);
+        CloseHandle(sei.hProcess);
+    }
+
+    // Give time for the theme to be applied
+    std::this_thread::sleep_for(std::chrono::milliseconds(1500));


Magic numbers without explanation. The sleep durations (10ms, 50ms, 100ms, 1500ms, 3000ms) are hardcoded without documentation explaining why these specific values were chosen. Consider extracting these to named constants with comments explaining the timing requirements.

niels9001 · 2026-04-01T07:42:49Z

Closing this PR as we have not figured out a good way to do this without 1) using a private API or 2) have proper support for multiple desktops.

Jaylyn-Barbee added 2 commits December 10, 2025 10:31

First try for theme switching

29db44a

Forgot resource strings and to update LoadSettings

c28a689

Jaylyn-Barbee marked this pull request as draft December 10, 2025 16:41

Jaylyn-Barbee added 2 commits December 15, 2025 11:47

XAML cleanup

b677135

theme is applying but settings window is opening

1dc0003

Jaylyn-Barbee added the Product-LightSwitch label Jan 5, 2026

Jaylyn-Barbee mentioned this pull request Jan 5, 2026

[Light Switch] Switch desktop wallpapers with the Light/Dark mode #42624

Merged

11 tasks

Jaylyn-Barbee closed this Jan 6, 2026

Jaylyn-Barbee reopened this Jan 8, 2026

Jaylyn-Barbee added 7 commits January 8, 2026 13:23

stablized theme switching, working on closing settings

23dcb54

Quickly closing the settings app now, still not too elegant

71d0256

Suppressing the settings app from opening when Theme swaps

fdd2514

ensure this new logic is only running when theme switching is enabled

7cfdedd

removing the older theme helper

2b1b42c

moved dupe code into a shared library

a2c3756

Merge branch 'main' into jay/theme-switching-new

ff74d64

Jaylyn-Barbee requested a review from Copilot January 9, 2026 18:13

Copilot started reviewing on behalf of Jaylyn-Barbee January 9, 2026 18:13 View session

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fixed themes link

97a361f

Jaylyn-Barbee requested a review from Copilot January 9, 2026 18:45

Copilot started reviewing on behalf of Jaylyn-Barbee January 9, 2026 18:45 View session

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clean up xaml

5aa4569

Jaylyn-Barbee requested a review from Copilot January 12, 2026 13:54

Copilot started reviewing on behalf of Jaylyn-Barbee January 12, 2026 13:54 View session

Copilot AI reviewed Jan 12, 2026

View reviewed changes

niels9001 closed this Apr 1, 2026


		var picker = new FileOpenPicker();

		var window = new Window();

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Summary of the Pull Request

PR Checklist

Notes

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karlingen commented Dec 27, 2025

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Jaylyn-Barbee commented Dec 10, 2025 •

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