Mobile is the new focus of a lot of attention and many Drupal themes are cropping up with a focus on making responsive designs easier. This theme was created as a demonstration that responsive design is easy to implement. The Fluid theme is comprised of nothing but an info file and a layout.css file (just like Drupal core's stark theme).
Fluid is an experiment to create a bare bones Drupal 7 theme with a fully functional responive layout built built with media query based break points to keep the layout friendly to displays of all sizes (including mobile). Experimenting with Fluid requires Compass and Susy.