prefer-effect

ImplementEffect subscribes when it mounts and unsubscribes when it unmounts. An ImplementLifecycle whose only job is to own a watch is doing that by hand:

// reported
ImplementLifecycle({ onMount: () => watch([theme], (t) => apply(t)) });

// what it means
ImplementEffect([theme], (t) => apply(t));

The rule only fires when the swap is a genuine simplification, so it stays quiet when the ImplementLifecycle has children, has an onUnmount, has any other prop, or uses the element onMount is handed.

It is also careful about which subscription it matches. watch and the standalone subscribe run the effect immediately with the current values, which is what a plain ImplementEffect does. A readable's own subscribe and onChange methods deliberately skip that first run, so the rule leaves them alone rather than rewriting them into an effect that behaves differently:

// not reported — onChange skips the current value, a plain effect would not
ImplementLifecycle({ onMount: () => query.onChange(refetch) });

ImplementEffect([query], refetch, { immediate: false }) is the equivalent of that last one, but the previous value onChange passes has no counterpart on an effect, so the rewrite is yours to make.