There are two areas from which you can control the defaults in MapViewer.
If you want to set specific mapping defaults, such as the hatch map default color spectrum file, you can use the Tools | Options | Default Settings command. This allows you to control default settings with the use of [.SET] files.
If you want to set line, fill, point symbol, and text defaults, along with MapViewer window features, you can use the Tools | Options command.