Why does a change in the website builder only show on some screen sizes?

Styles in the website builder cascade down. A change made in desktop mode applies to desktop and everything smaller. A change made in tablet mode applies to tablet and everything smaller (mobile), but not to desktop.

So to add or remove padding only on smaller screens, switch to tablet mode first and make the change there, and the desktop version stays as it is. Set values like padding for the selected section in the sidebar. Always check mobile too, since padding that looks fine on desktop may be too much on a phone.