Icons in ~/.icons (or rather ~/.local/share/icons for the standard-compliant path) can be used to complete a theme or install a new one too, you don’t specifically have to put them in /usr/share/icons. The only difference is that they’ll be usable by the related user only and not everyone.
If you download an icon theme and are not too sure how to use it or not tech-savvy enough to get admin rights to put its files in the system path, it’s perfectly fine to install it in ~/.local/share/icons.
The same goes for Gtk themes, which are in /usr/share/themes (system-wide) or ~/.local/share/themes (user-specific − ~/.themes is valid too).
@wimpy
Its incredible the only way for ubuntu-mate to look polished is using: “Ambient-mate” paired with Ambient-mate icons (blue or green).
Downloading GTK 3 themes for mate, suck either is controls, windows, menubar, logo, etc.
Plase fix this for zesty zapus or make more default themes (which is not a solution), this needs attention because basically we are stuck with a default theme and icons that are “polished” the rest of the gnome-look themes suck.
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