Theming settings for the splashscreen + Flowers theme

I've added a theming setting to the preferences, all it does now is switch around some of the css using ng-class. I've also included a theme using Cass's flowery background picture that a lot of people, especially the Chinese, liked a lot!

If you don't want to add the Flowers theme I'll make another pull request with only the feature, without the theme, then Cass can maybe add a dark theme or something else.

![](http://i61.tinypic.com/35ibwqq.jpg)