Mylo-Xylo icons are round colorful icons designed for Gnome desktop and based on All-black icon set. It is under development and should not expect it as complete icon set but creator mentioned that version 1.0 will be complete set. You can also contribute to this project by going to source link and contact to creator or report any bug/missing icon to creator. You can use Unity Tweak Tool, Gnome-tweak-tool or Ubuntu-Tweak to change icons.
To install Mylo-Xylo icons in Ubuntu 15.04/14.10/14.04/12.04/Linux Mint 17.1/17/13 open Terminal (Press Ctrl+Alt+T) and copy he following commands in the Terminal:
To install Mylo-Xylo icons in any Ubuntu/Linux Mint version open Terminal (Press Ctrl+Alt+T) and copy the following commands in the Terminal:
Source: Mylo Xylo