Getting one screen right is craft. Getting the hundredth screen right, built by someone who wasn't in the room for the first, is a system. This section is about writing your decisions down so they last: in tokens, in component APIs and in rules that other people can follow without having to ask you.
A good system makes the right thing easy and the wrong thing obvious. Themes become a token swap instead of a second stylesheet. Components are designed like APIs, because other people will have to learn them. And when the system starts to drift, one small override at a time, someone notices before it ships.
A color value, what it means, and where it's used are three different decisions. Keep them apart.
Light, dark and a second brand should be a token swap, not a second stylesheet.
Props are an interface other people have to learn. Design them like one.
A component that takes thirty props is a component nobody can extend.
Every one-off className tweak is a variant the system doesn't know about yet.
Some screens need room to breathe and some need to fit a lot. Make it a setting, not a fork.
Build the hard part once, then dress it any way you want.
Systems drift one small change at a time. Make drift visible before anyone has to notice it.