Build you have downloaded contains several folders, each for one color scheme.
So, you shouldn't think "What class should i use to switch theme color scheme?", just take folder that has name of color you need.
To set boxed layout add .pi-fixed class to body element.
To set another body background add one of the classes listed below to body element.
Don't try to set body background in full width mode. Use sections background if you want to set background in full width mode.