For image assets, meegotouch-theme-darko is the package you'd want to look into. I'll push it to the nemomobile github later on this evening. We don't currently have a maintainer there, so I guess you'd be it.

In terms of layout suggestions, I'd much rather that we have people to implement them - as we're desperately short of UI developers. If you think you can try your hand at writing QML, that's definitely useful.

Pop onto #nemomobile on irc.freenode.net, and start getting to know things and folks