Thanks for the reply. I guess it's because everything I'm trying to do is less specific than the default skin. I'm trying to have as few borders/backgrounds/decorative stuff as possible.

The AO3 doesn't have an option to start with a stripped-down layout, does it? Like LJ/DW's option to not use a style's default CSS?

I've added the CSS to the post, if you'd be willing to take a look at it? It's still a WIP so I'm sure there's some redundancy in there that I'll be stripping out later.