uxtheme: Add ShouldSystemUseDarkMode.

dlls/uxtheme/system.c

@@ -1256,3 +1256,20 @@ BOOL WINAPI ThemeHooksRemove(void)
     UnregisterUserApiHook();
     return TRUE;
 }
+
+/**********************************************************************
+ *      ShouldSystemUseDarkMode                           (UXTHEME.138)
+ *
+ * RETURNS
+ *     Whether or not the system should use dark mode.
+ */
+BOOL WINAPI ShouldSystemUseDarkMode(void)
+{
+    DWORD light_theme = TRUE, light_theme_size = sizeof(light_theme);
+
+    RegGetValueW(HKEY_CURRENT_USER,
+                 L"Software\\Microsoft\\Windows\\CurrentVersion\\Themes\\Personalize",
+                 L"SystemUsesLightTheme", RRF_RT_REG_DWORD, NULL, &light_theme, &light_theme_size);
+
+    return !light_theme;
+}

dlls/uxtheme/tests/system.c

@@ -48,6 +48,7 @@ static HDC (WINAPI *pGetBufferedPaintTargetDC)(HPAINTBUFFER);
 static HRESULT (WINAPI *pGetBufferedPaintTargetRect)(HPAINTBUFFER, RECT *);
 static HRESULT (WINAPI *pGetThemeIntList)(HTHEME, int, int, int, INTLIST *);
 static HRESULT (WINAPI *pGetThemeTransitionDuration)(HTHEME, int, int, int, int, DWORD *);
+static BOOL (WINAPI *pShouldSystemUseDarkMode)(void);
 
 static LONG (WINAPI *pDisplayConfigGetDeviceInfo)(DISPLAYCONFIG_DEVICE_INFO_HEADER *);
 static LONG (WINAPI *pDisplayConfigSetDeviceInfo)(DISPLAYCONFIG_DEVICE_INFO_HEADER *);
@@ -74,6 +75,8 @@ static void init_funcs(void)
     HMODULE gdi32 = GetModuleHandleA("gdi32.dll");
     HMODULE uxtheme = GetModuleHandleA("uxtheme.dll");
 
+    pShouldSystemUseDarkMode = (void *)GetProcAddress(uxtheme, MAKEINTRESOURCEA(138));
+
 #define GET_PROC(module, func)                       \
     p##func = (void *)GetProcAddress(module, #func); \
     if (!p##func)                                    \
@@ -2597,6 +2600,35 @@ static void test_theme(void)
     DestroyWindow(hwnd);
 }
 
+static void test_ShouldSystemUseDarkMode(void)
+{
+    DWORD light_theme, light_theme_size = sizeof(light_theme), last_error;
+    BOOL result;
+    LSTATUS ls;
+
+    if (!pShouldSystemUseDarkMode)
+    {
+        win_skip("ShouldSystemUseDarkMode() is unavailable.\n");
+        return;
+    }
+
+    ls = RegGetValueW(HKEY_CURRENT_USER,
+                      L"Software\\Microsoft\\Windows\\CurrentVersion\\Themes\\Personalize",
+                      L"SystemUsesLightTheme", RRF_RT_REG_DWORD, NULL, &light_theme, &light_theme_size);
+    if (ls == ERROR_FILE_NOT_FOUND)
+    {
+        light_theme = 1;
+        ls = 0;
+    }
+    ok(ls == 0, "RegGetValue failed: %ld.\n", ls);
+
+    SetLastError(0xdeadbeef);
+    result = pShouldSystemUseDarkMode();
+    last_error = GetLastError();
+    ok(last_error == 0xdeadbeef, "ShouldSystemUseDarkMode set last error: %ld.\n", last_error);
+    ok(result == !light_theme, "Expected value %d, got %d.\n", !light_theme, result);
+}
+
 START_TEST(system)
 {
     ULONG_PTR ctx_cookie;
@@ -2622,6 +2654,7 @@ START_TEST(system)
     test_DrawThemeBackgroundEx();
     test_GetThemeBackgroundRegion();
     test_theme();
+    test_ShouldSystemUseDarkMode();
 
     if (load_v6_module(&ctx_cookie, &ctx))
     {

dlls/uxtheme/uxtheme.spec

@@ -42,6 +42,7 @@
 61 stdcall OpenThemeDataEx(ptr wstr long)
 62 stub -noname ServerClearStockObjects
 63 stub -noname MarkSelection
+138 stdcall -noname ShouldSystemUseDarkMode()
 
 # Standard functions
 @ stdcall BeginBufferedAnimation(ptr ptr ptr long ptr ptr ptr ptr)