GH-1790 do not apply system theme on launch if it is selected

This prevents some ugly colors to show up on macOS in most cases.
It still looks ugly right after you switch to the it though.

application/themes/SystemTheme.h

@@ -7,6 +7,7 @@ class SystemTheme: public ITheme
 public:
 	SystemTheme();
 	virtual ~SystemTheme() {}
+	void apply(bool initial) override;
 
 	QString id() override;
 	QString name() override;