Custom Breakpoints When Needed

The v3 answer, `theme.extend.screens`, no longer exists — there is no config object to extend. A breakpoint in v4 is just another theme token: declare `--breakpoint-3xl: 120rem;` inside `@theme` and you get `3xl:` (plus `max-3xl:` and every other variant that keys off it) for free. The rule underneath is unchanged and is the reason this principle exists: a raw `min-[960px]:` is a magic number with no name, so the second person who needs that breakpoint guesses 961 or 959 and the layout now breaks in a one-pixel band.

Name it once, use the name everywhere. Two v4 specifics worth knowing: because breakpoints are ordinary custom properties, they are sorted by value rather than by declaration order, so a new token slots into the right place in the cascade automatically; and `--breakpoint-*: initial;` wipes the built-in scale if you want only your own names.