force Night Mapping theme on Mac Mojave with Dark Theme (#8309)

* force Night Mapping theme on Mac Mojave with Dark Theme

otherwise QGIS is pretty unusable
see #20195

* [themes] add automatic mode to base theme on OS theme

only effective on Mac OS for now

* Revert "add auto theme based on OS to choose default / dark theme"

This reverts commit d012451cadb4653421eb49711ac99ed48e064622.

* apply auto theme only for Qt < 5.12

* Update src/app/qgisapp.cpp

* better doc

src/app/qgisapp.cpp

@@ -2002,8 +2002,8 @@ int QgisApp::dockedToolbarIconSize( int standardToolbarIconSize ) const
 void QgisApp::readSettings()
 {
   QgsSettings settings;
-  QString themename = settings.value( QStringLiteral( "UI/UITheme" ), "default" ).toString();
-  setTheme( themename );
+  QString themeName = settings.value( QStringLiteral( "UI/UITheme" ), "default" ).toString();
+  setTheme( themeName );
 
   // Read legacy settings
   readRecentProjects();
@@ -3208,7 +3208,21 @@ void QgisApp::setTheme( const QString &themeName )
   for the user to choose from.
   */
 
-  QgsApplication::setUITheme( themeName );
+  QString theme = themeName;
+#ifdef Q_OS_MAC
+#if QT_VERSION < QT_VERSION_CHECK( 5, 12, 0 )
+  if ( theme == QStringLiteral( "default" ) &&
+       QgsGui::instance()->nativePlatformInterface()->hasDarkTheme() )
+  {
+    QString darkTheme = QStringLiteral( "Night Mapping" );
+    if ( QgsApplication::uiThemes().contains( darkTheme ) )
+      theme = darkTheme;
+  }
+#endif
+#endif
+
+  QgsApplication::setUITheme( theme );
+
   //QgsDebugMsg("Setting theme to \n" + themeName);
   mActionNewProject->setIcon( QgsApplication::getThemeIcon( QStringLiteral( "/mActionFileNew.svg" ) ) );
   mActionOpenProject->setIcon( QgsApplication::getThemeIcon( QStringLiteral( "/mActionFileOpen.svg" ) ) );

src/app/qgsoptions.cpp

@@ -566,7 +566,7 @@ QgsOptions::QgsOptions( QWidget *parent, Qt::WindowFlags fl, const QList<QgsOpti
   QString name = mSettings->value( QStringLiteral( "/qgis/style" ) ).toString();
   cmbStyle->setCurrentIndex( cmbStyle->findText( name, Qt::MatchFixedString ) );
 
-  QString theme = QgsApplication::themeName();
+  QString theme = mSettings->value( QStringLiteral( "UI/UITheme" ), QStringLiteral( "auto" ) ).toString();
   cmbUITheme->setCurrentIndex( cmbUITheme->findText( theme, Qt::MatchFixedString ) );
 
   mNativeColorDialogsChkBx->setChecked( mSettings->value( QStringLiteral( "/qgis/native_color_dialogs" ), false ).toBool() );
@@ -1235,7 +1235,7 @@ void QgsOptions::uiThemeChanged( const QString &theme )
   if ( theme == QgsApplication::themeName() )
     return;
 
-  QgsApplication::setUITheme( theme );
+  QgisApp::instance()->setTheme( theme );
 }
 
 void QgsOptions::mProjectOnLaunchCmbBx_currentIndexChanged( int indx )

src/native/mac/qgsmacnative.h

@@ -38,6 +38,8 @@ class NATIVE_EXPORT QgsMacNative : public QgsNative
     QgsNative::Capabilities capabilities() const override;
     QgsNative::NotificationResult showDesktopNotification( const QString &summary, const QString &body, const NotificationSettings &settings ) override;
 
+    bool hasDarkTheme() override;
+
   private:
     class QgsUserNotificationCenter;
     QgsUserNotificationCenter *mQgsUserNotificationCenter = nullptr;

src/native/mac/qgsmacnative.mm

@@ -118,3 +118,8 @@ - ( BOOL )userNotificationCenter:( NSUserNotificationCenter * )center shouldPres
   result.successful = true;
   return result;
 }
+
+bool QgsMacNative::hasDarkTheme()
+{
+  return ( NSApp.effectiveAppearance.name != NSAppearanceNameAqua );
+}

src/native/qgsnative.h

@@ -135,6 +135,11 @@ class NATIVE_EXPORT QgsNative : public QObject
      */
     virtual void setApplicationBadgeCount( int count );
 
+    /**
+     * Returns true if the operating system is set to utilise a "dark" theme.
+     */
+    virtual bool hasDarkTheme() {return false;}
+
     /**
      * Notification settings, for use with showDesktopNotification().
      */