PowerToys/src/modules/LightSwitch/LightSwitchService/ThemeHelper.cpp

#include <windows.h>
#include "ThemeHelper.h"

// Controls changing the themes.

void SetAppsTheme(bool mode)
{
    HKEY hKey;
    if (RegOpenKeyEx(HKEY_CURRENT_USER,
                     L"Software\\Microsoft\\Windows\\CurrentVersion\\Themes\\Personalize",
                     0,
                     KEY_SET_VALUE,
                     &hKey) == ERROR_SUCCESS)
    {
        DWORD value = mode;
        RegSetValueEx(hKey, L"AppsUseLightTheme", 0, REG_DWORD, reinterpret_cast<const BYTE*>(&value), sizeof(value));
        RegCloseKey(hKey);

        SendMessageTimeout(HWND_BROADCAST, WM_SETTINGCHANGE, 0, reinterpret_cast<LPARAM>(L"ImmersiveColorSet"), SMTO_ABORTIFHUNG, 5000, nullptr);

        SendMessageTimeout(HWND_BROADCAST, WM_THEMECHANGED, 0, 0, SMTO_ABORTIFHUNG, 5000, nullptr);
    }
}

void SetSystemTheme(bool mode)
{
    HKEY hKey;
    if (RegOpenKeyEx(HKEY_CURRENT_USER,
                     L"Software\\Microsoft\\Windows\\CurrentVersion\\Themes\\Personalize",
                     0,
                     KEY_SET_VALUE,
                     &hKey) == ERROR_SUCCESS)
    {
        DWORD value = mode;
        RegSetValueEx(hKey, L"SystemUsesLightTheme", 0, REG_DWORD, reinterpret_cast<const BYTE*>(&value), sizeof(value));
        RegCloseKey(hKey);

        SendMessageTimeout(HWND_BROADCAST, WM_SETTINGCHANGE, 0, reinterpret_cast<LPARAM>(L"ImmersiveColorSet"), SMTO_ABORTIFHUNG, 5000, nullptr);

        SendMessageTimeout(HWND_BROADCAST, WM_THEMECHANGED, 0, 0, SMTO_ABORTIFHUNG, 5000, nullptr);
    }
}

// Can think of this as "is the current theme light?"
bool GetCurrentSystemTheme()
{
    HKEY hKey;
    DWORD value = 1; // default = light
    DWORD size = sizeof(value);

    if (RegOpenKeyEx(HKEY_CURRENT_USER,
                     L"Software\\Microsoft\\Windows\\CurrentVersion\\Themes\\Personalize",
                     0,
                     KEY_READ,
                     &hKey) == ERROR_SUCCESS)
    {
        RegQueryValueEx(hKey, L"SystemUsesLightTheme", nullptr, nullptr, reinterpret_cast<LPBYTE>(&value), &size);
        RegCloseKey(hKey);
    }

    return value == 1; // true = light, false = dark
}

bool GetCurrentAppsTheme()
{
    HKEY hKey;
    DWORD value = 1;
    DWORD size = sizeof(value);

    if (RegOpenKeyEx(HKEY_CURRENT_USER,
                     L"Software\\Microsoft\\Windows\\CurrentVersion\\Themes\\Personalize",
                     0,
                     KEY_READ,
                     &hKey) == ERROR_SUCCESS)
    {
        RegQueryValueEx(hKey, L"AppsUseLightTheme", nullptr, nullptr, reinterpret_cast<LPBYTE>(&value), &size);
        RegCloseKey(hKey);
    }

    return value == 1; // true = light, false = dark
}