You can freely style your site the way you want. No wrong way really as long it's not the original because it will get overwritten with updates.
Create a new mix
Well, that's the only good way to go. So your changes will not be overwritten by an update. If the width is not respected after a change in "Mix" empty the caches, it should work.
Correct, but I did have to update the width in body for the site to enlarge.