i think i figured out the problem.
* the AppDefaults section, when found in any skin that's not actually the default skin (the underlying backup skin), is removed.
* where that key is actually used, it's specified to use the default only. (this seems to be a mistake)
this means that the only other way to change this icon is to make changes to digsby/res/skins/default/skin.yaml. this is something i really don't like users doing, because it gets reverted on updates and things like that and people get angry about it. for that reason, i'm not sure if this is better or worse than changing the icon. probably about the same ;-p
(this is how it should be done, but as i said that won't work in this case)