Shadow icon theme is a new comer for Linux desktop, it looks beautiful with all kind of themes. It is meant to be modern clean and customizable, the primary color of this set most likely bluish and many apps icons are in round shape. So basically this theme is mixture of round and normal (square) shape icons, lets see where this theme will head in the future, it should choose shape what users asks. As creator mentioned this icon theme is his first so please bare any bugs or missing icons. You can report bugs or suggest new icons to include in this set via this link. You can use Unity Tweak Tool, Gnome-tweak-tool to change themes/icons.