Custom Forms

I've seen this asked a few places, but I'm not finding any solutions. We need to have our own signup form—or at least one that we can really style to meet our UX requirements, which doesn't seem to be supported. It appears to be recommended that one then create their own form and use the v3 API. Cool, the API supports OAuth, great, but not the client credentials grant. How does my server post, then? The end user shouldn't be involved in authorizing anything, as they're just signing up for a newsletter.   

I'm missing something because posting a new contact to a list shouldn't be this hard.

~ Lacey