Icon themes can be modified using the icon.ini file. Changes to this file, however, could cause unexpected behavior when the IconSettings/Size does not exist in the IconSettings/Sizes array. There is pre-existing code that is supposed to account for this scenario but instead it triggers an "undfined index" error because the fallback is looking for a numbered key.
To trigger the error, modify the icon.ini that is in use for your siteaccess and change the default size to something that does not exist.
I have a fix proposed here: https://github.com/ezsystems/ezpublish-legacy/pull/1290