A GTK theme is a collection of visual elements which provides the same look and feel for across applications. Most of the extensions with GUI are using GTK+ graphical user interface which have to be setup properly to avoid missing icons on push buttons, displaying wrong characters, etc. Desktop environments like Xfce4 and LXDE are doing it, but in other cases it must be done manually.
An icon theme is basically a collection of cathegorised icons in different size and in scalable format in most cases.
There are several generic icon themes available having nearly complete icon sets, suitable in most situations. Safe choise to start with gnome-icon-theme or lxde-icon-theme which are in the TC repository, but you can try others as well.
There are special purpose icon themes providing less icons. Using such may result missing application icons.
Any number of icon themes can be installed, but only one used at the same time.
GTK theme is defined by .gtkrc-2.0 file in the users home directory. As a minimum it must contain icon theme name, like:
for lxde-icon-theme or
for gnome-icon-theme. Other parameters can be added to the file:
While configuration file is human readable and can be edited manually, usually it is handled by programs. You can include your own configuration file to add parameters not handled by such program:
Warning: Desktops, like LXDE and Xfce4 or extensions, like CPManFM take care of theme setting and provide .gtkrc-2.0 Check existence before installing your own, do not overwrite it. Install your own carefuly and only if really necessary!
Additionally to the icon theme following extensions must be installed:
Optionally librsvg.tcz maybe required to handle scalable pixmaps properly.
In Xfce4 you can use the built-in setting tool. Otherwise use lxappearance to configute GTK theme. It is developed for LXDE, but in fact a small desktop-independent theme switcher for GTK+ usable stand alone.