GH-903 simple theme switching and dark theme

application/themes/SystemTheme.cpp

@@ -0,0 +1,28 @@
+#include "SystemTheme.h"
+#include <QApplication>
+#include <QStyle>
+
+SystemTheme::SystemTheme()
+{
+	systemPalette = QApplication::style()->standardPalette();
+}
+
+QString SystemTheme::id()
+{
+	return "system";
+}
+
+QString SystemTheme::name()
+{
+	return QObject::tr("System");
+}
+
+QPalette SystemTheme::colorScheme()
+{
+	return systemPalette;
+}
+
+QString SystemTheme::appStyleSheet()
+{
+	return QString();
+}