There isn't a mobileme theme. The first message in this thread is a proposal for a theme, but it never got done. Themes can be created by editing the CSS and images in the public/assets/themes/default folder.
We only have one theme, which is the default theme, but we are open to submissions from the community. You can start by changing the ExtJS theme, which is the javascript library that provides most of the widgets used in OpenGoo. There are already many ExtJS themes on the web. Check in the ExtJS site or Google for it.