How to make your pages not 100% wide by default

Community Novice

I have been watching different implementations of CANVAS and I see that the ideal is to have the maximum width of the pages not at 100%, but at a percentage like 80%. This way, it doesn't look bad when the screen resolution is FULLHD.

I haven't found how to change this parameter.

I have seen implementations that have a different width than 100%

For example here: Learn More About Images and Media and their use in Canvas: Visual Design in Canvas 

Default in pages is not 100%

And other implementations, like mine and this one, where the width is always 100% and I don't know where to change that parameter.

100 percent wide

I would like a 80% by default.