Modularize logo and theme style

Changed the name of the theme style file to theme.css.
This way, any config that defines a new theme.css will
overwrite the default.

Make the logo a background image instead of an <img>
With a background image, use logo.png and logo@2x.png for
high resolution (retina) screens.

Put assets/styles/theme.css in your project to overwrite
the default

Put assets/images/logo.png and assets/images/logo@2x.png
in your project to overwrite the default