An icon theme is nothing but a group of icons that have the same kind of feel and look about them. It is up to the user to select the particular icon theme that he/she is interested in using. Thereafter all the apps make use of the icons from that theme. The icon theme’s initial user is actually the desktop file specification’s icon field. However, it can have other uses like in case of mime type icons too. From the perspective of a programmer, an icon theme is nothing more than a mapping method. Once provided a particular set of directories to pick up icons from, and a particular theme name, an icon filename is mapped from an icon name and a nominal icon size.

Let’s go over some important definitions related to icon themes now, starting with:
The icon theme – As also explained above, an icon theme is simply a named set of icons which is utilized for mapping from an icon’s name and size, to a particular file name. Icon themes may also inherit from other icon themes, as a method of extending them.
Icon file – It is basically an image which can be loaded and made use of in the form of an icon. The supported image file formats in this regard are SVG, XPM and PNG. SVG is the recommended format for the vectorized icons and PNG is best for bitmap format. XPM is supported only in order to provide backward compatibility. Newer themes are recommended to not make use of XPM files.
Base directory – The set of directories in which one can look up different icons and themes are referred to as base directories. All the icon themes get stored in the subdirectories of these base directories.