This page displays a list of the icon sets you have installed. Icon Sets are collections of icons that can be used for toolbars and menus. Opus comes with one set built-in and you can add as many additional sets as you like.
You can use the controls on this page to change the order icon sets are used in. Toolbar buttons and menus that use icons usually refer to the icon by name (for example, the Copy Files command would use the icon called "copy"), and icon sets specify the name of all the icons they contain. When Opus needs to display an icon it starts at the first icon set in the list and looks for an icon with the desired name. If that set doesn't contain the icon it moves on to the next set, and so on. So it's generally possible to change the icons in use by your toolbars simply by changing the order that Opus looks through the icon sets for the desired icon.
The toolbar buttons at the top of the page let you manipulate the icon sets:
(move set up in the order of
precedence), 
(move set down), 
(import a set from disk), 
(delete a set) and 
(refresh the list of sets).
The higher up the list a set is, the sooner it is checked for a given icon. In the above screen shot, the Default Icon Set for Opus 10 is at the top, and so the default toolbars will be using this - but if you selected the Directory Opus 9 Icon Set and moved it to the top, then the default toolbars would switch back to using Opus 9 icons.
The bottom part of this page displays a preview of all the icons contained in the currently selected icon set.
You can download additional icon sets from the Resource Centre. If you want to create your own icon sets, the Save Icon Set Template command in the File menu can get you started. It produces a template of the standard icon names - you will need to provide your own images however! See the Icon Set Format Specification for more information on creating your own sets.