GH-903 simple theme switching and dark theme

application/themes/DarkTheme.h

@@ -0,0 +1,14 @@
+#pragma once
+
+#include "ITheme.h"
+
+class DarkTheme: public ITheme
+{
+public:
+	virtual ~DarkTheme() {}
+
+	QString id() override;
+	QString name() override;
+	QString appStyleSheet() override;
+	QPalette colorScheme() override;
+};