Qt: Use "fusion" style when dark mode is activated on Windows 10/11

src/qt/qt_main.cpp

@@ -154,9 +154,45 @@ main_thread_fn()
 
 static std::thread *main_thread;
 
+#ifdef Q_OS_WINDOWS
+static bool is_light_theme() {
+    // based on https://stackoverflow.com/questions/51334674/how-to-detect-windows-10-light-dark-mode-in-win32-application
+
+    // The value is expected to be a REG_DWORD, which is a signed 32-bit little-endian
+    auto buffer = std::vector<char>(4);
+    auto cbData = static_cast<DWORD>(buffer.size() * sizeof(char));
+    auto res = RegGetValueW(
+        HKEY_CURRENT_USER,
+        L"Software\\Microsoft\\Windows\\CurrentVersion\\Themes\\Personalize",
+        L"AppsUseLightTheme",
+        RRF_RT_REG_DWORD, // expected value type
+        nullptr,
+        buffer.data(),
+        &cbData);
+
+    if (res != ERROR_SUCCESS) {
+        return 1;
+    }
+
+    // convert bytes written to our buffer to an int, assuming little-endian
+    auto i = int(buffer[3] << 24 |
+        buffer[2] << 16 |
+        buffer[1] << 8 |
+        buffer[0]);
+
+    return i == 1;
+}
+#endif
+
 int
 main(int argc, char *argv[])
 {
+#ifdef Q_OS_WINDOWS
+    qputenv("QT_QPA_PLATFORM", "windows:darkmode=2");
+    if (!is_light_theme()) {
+        qputenv("QT_STYLE_OVERRIDE", "fusion");
+    }
+#endif
 #if QT_VERSION < QT_VERSION_CHECK(6, 0, 0)
     QApplication::setAttribute(Qt::AA_DisableHighDpiScaling, false);
     QApplication::setAttribute(Qt::AA_UseHighDpiPixmaps);