Learn Croatian: Prvo stavljam puder na lice. - First I put foundation on my face.

Yes, in everyday Croatian, puder is used as a general term for base makeup. Technically, liquid foundation is tekući puder (liquid powder), while dry powder is puder u prahu (loose powder) or puder u kamenu (pressed powder). However, most people just say puder for whatever foundation they use.

Why use the imperfective verb stavljam instead of the perfective stavim?

The imperfective stavljati is used here because the speaker is describing a routine, a general habit, or narrating an ongoing step-by-step process (like in a makeup tutorial). If they were stating an intention to do this as a single, completed action right now, they would use the perfective: Stavit ću puder (I will put foundation on).

Why is lice in the accusative case here instead of the locative?

The preposition na takes the accusative case when it expresses motion or direction toward a destination. Because you are actively moving the makeup onto your face, it's na lice. If you were describing where the makeup already rests without motion, you would use the locative case: Puder je na licu (The foundation is on the face).

Is prvo an adjective agreeing with a neuter noun here?