Switch to classic if theme set to auto and high contast mode is on.

The light and dark theme don't respond to Windows's high contrast
accessibility mode, so when the theme is set to "auto", we
default to "classic" instead of "light".

Fixes #5044

src/gui/Application.cpp

@@ -147,6 +147,11 @@ void Application::applyTheme()
     auto appTheme = config()->get(Config::GUI_ApplicationTheme).toString();
     if (appTheme == "auto") {
         appTheme = osUtils->isDarkMode() ? "dark" : "light";
+#ifdef Q_OS_WIN
+        if (winUtils()->isHighContrastMode()) {
+            appTheme = "classic";
+        }
+#endif
     }
 
     if (appTheme == "light") {

src/gui/osutils/winutils/WinUtils.cpp

@@ -105,3 +105,9 @@ bool WinUtils::isCapslockEnabled()
 {
     return GetKeyState(VK_CAPITAL) == 1;
 }
+
+bool WinUtils::isHighContrastMode() const
+{
+    QSettings settings(R"(HKEY_CURRENT_USER\Control Panel\Accessibility\HighContrast)", QSettings::NativeFormat);
+    return (settings.value("Flags").toInt() & 1u) != 0;
+}

src/gui/osutils/winutils/WinUtils.h

@@ -36,6 +36,7 @@ public:
     bool isLaunchAtStartupEnabled() const override;
     void setLaunchAtStartup(bool enable) override;
     bool isCapslockEnabled() override;
+    bool isHighContrastMode() const;
 
 protected:
     explicit WinUtils(QObject* parent = nullptr);