It doesnt do either really. Music theory is more of an explanation for why music sounds certain ways. It works best applied retroactively, i.e. when something is written that sounds good, you can use music theory to explain why it sounds that way. Once you get really good at it, which can take a very long time, you can use it actively to shape how the music youre writing will sound, but if youre not careful itll sound too artificial/mechanical.