I don't think that you can't make a popular RP just because you are not great at CSS or are not well known. Often times people will join just because its an interesting idea. CSS is definitely a great way of grabbing people's attention when they first click on the thread, but if they are just joining because the CSS looks cool and professional, they might not stick around if the idea is not solid and interesting enough.
I'm not great at CSS either, for the most part all I do is take existing CSS from other people's posts, then modify it heavily by changing the colors, background, etc, most of which is pretty simple.
In short, I'd just say go with it. It's intimidating, but you'd be surprised at how much attention you could get.