Yelp customers don't want theme-able, if I'm browsing five restaurants closest to me to decide which ones to go to in the next 10 minutes, I want to see the type of cuisine, hours, menus, etc. in a consistent way instead of playing "find Waldo" with whatever artistic genius designed that specific theme.
Sure, but local businesses will still want control over their main web address. I don't think very many are confident enough to simply redirect you to their Yelp page.
Google runs a GYBO (Get Your Business Online) program doing that, down to domain name, in an effort to upsell things like GApps once the small business is ready, I'm not sure how popular it is.
