The Icon Set system allows you to create a custom icon set that can “drop in” and replace, or augment, the internal set of toolbar icons. A list of installed icon sets can be viewed in Preferences on the Toolbars / Icons page.
An Icon Set is distributed in a .dis file, which is a normal ZIP file with a .dis extension. Inside the .dis file is an XML file that defines the icon set, and one or more image files that contain the actual image data for the icons. Any image format supported by Directory Opus can be used.
More:
Icon Set XML Definition File
Icon Sizes
Icon Names
Icon Display Names
Icon Categories
DPI aware Icon Sets
Localization
Icon Images