There are plenty of cool icon sets available for Linux, Square icon set which was released last year is great competitor to other nice icons. It is elegant, well polished and looks great with all kind of themes but with Unity you must use dark panel theme because its status icons are kind of white/gray. This icon set is compatible with Unity, Gnome, Xfce, Lxde, Mate, Cinnamon desktops as well as KDE and it contains more than 15,000 icons, now this set offers two theme Light version for light color themes and Dark version for darker themes. If you found any missing icon in this set then report to creator. Ambiance blackout theme used in screenshots. You can use Unity Tweak Tool, Gnome-tweak-tool or Ubuntu-Tweak to change icons.
To install Square icons in Ubuntu 15.04/15.10/14.04/12.04/Linux Mint 17.x/17/13 open Terminal (Press Ctrl+Alt+T) and copy the following commands in the Terminal:
To install Square icons in any Debian/Ubuntu/Linux Mint version open Terminal (Press Ctrl+Alt+T) and copy the following commands in the Terminal:
Update: Huge icons issue fixed.