I would like to know where are the GTK+ Themes icons stored?For example, where are the ambiance theme GUI icons stored?(Like the go back, go up, search icons)
The user added icons are in the user's home folder in a hidden folder:
The installed icons through APT aswell as the defualt icons are in:
The first can only be used by the user that has them, the second can be used system wide.The icons you mentioned for example are in the Humanity folder under usr/share/icons/Humanity/actions/XX (xx=size) most are in the 48 size folder and are in both .png and svg file formats and some other in ubuntu-mono-dark.
To edit them you need to use
I would like to add that to anybody who wants to have a look at the icons GNOME uses, he should take a look at GNOME's iconfinder profile. Most of the icons,including a couple GTK+ ones are available at that website.