Work is moving on to the detailed level of interface look and feel. Most, if not all, Substrate installations will have to be adapted to the graphical profiles of specific customers. Given that, it makes sense to think about the user-interface in terms of skins.
Here is a suggested dark skin that uses a gentle background pattern and semi-transparency on some widget borders to give an organic feel.
There are also some new interaction features in this set of sketches, including the dual-viewport mode that may come in handy for certain navigation and editing tasks.
For some demo purposes, it will certainly be necessary to have a lighter skin. Here is a sketch of a clean and basic treatment.