On first launch of the app, you can sample all the available themes before you commit to one. Don't worry, you can always go back and change it later by opening the slide-out menu.
A little further down the slide-out menu, you can open other Yahoo apps if you have them (or download them if you don't), and you can switch the theme or access settings.
When you decide to write an e-mail, Yahoo Mail goes back to basics with a black-on-white screen. You also can snap a photo or choose one from your library if you want to attach an image.