I still like more your first mockup,
I guessed you meant the one from Thomas :)
from which I would remove the Style tab and just have the theme customization in a separate window, like today. How does that sound?
The big question is whether we also want metathemes to include background and fonts. If this is the case (what it seems to be today in the form of suggestions), having two capplets for the same thing seems strange. And: how would a user know that changing e.g. the icon theme requires him to go to Themes and not to Appearance?