dnd 5e – Can you cast spells requiring sight on targets that aren’t perfectly defined?

If you can see it, you can target it

Page 204 of the Player’s Handbook, in the Spellcasting chapter, says:

To target something, you must have a clear path to it, so it can’t be behind total cover.

That, to my mind, is what is meant by any spell description that says “a target you can see”. You don’t need to be able to read their facial expression or make out what color shirt they are wearing. You just need to be able to define where target is and have nothing blocking your view.

It means they can’t be behind total cover or around a corner, etc.

In your situation, you could see the target, even if only as a silhouette, so you can target them with spells that require being able to “see” them.