I’ve been searching for these for quite a while actually, ever since I saw this.
(try making your browser smaller to the approximate width of a mobile phone after you open this page and see what happens)
I figured that someone had to be working on a theme somewhere that didn’t need mobile themes etc, but changed its appearance according to the size of the screen you’re using to view it.
So finally some themes are starting to pop up here and there.
Here are some of my favorites: