The Darkness spell says:
This also suppresses magical light of your darkness spell's level or lower.
Assuming it isn't heightened, this means Darkness is a 2nd level spell.
The rules for Cantrips say:
A cantrip is always automatically heightened to half your level, rounded up.
The rules for Heightened Spells say:
When you heighten your spell, the spell’s level increases to match the higher level of the spell slot you’ve prepared it in or used to cast it.
So a 5th level caster casting Light would cast it heightened to 3rd level, meaning it counts as a 3rd level spell.
Is this to say that a 5th level caster can counteract Darkness (at its base level) with a cantrip?
Best Answer
Yes
You already cite all the relevant rules.
Maybe you are wondering if it is appropriate that a simple cantrip, cast by a high-enough level caster, will overcome a leveled spell. Narratively you can look at it this way: with the rule of automatically heightening their spell level, the design of cantrips in PF2E has them also be an expression of the caster's power, and it is that power that overcomes the spell.