Editing the Subscription popup

Hi again Ghost forum,
I would love to know if it is at all possible to customize the subscription button/settings using a code injection or other method rather than editing the theme. I use dawn, and I don’t need anyone to be able to “sign in” or anything when they click subscribe, only put in their email address to get added to the newsletter for free. Does anyone know how to delete that, or maybe edit the whole popup so that I could add some text about what they are subscribing to? Thanks so much in advance.