Any icon pack (not themes like Cyanogen Mod themes for example) can only apply to launchers and apps not the android system itself
(except that the system supports icon packs which is not common in official ROMs)
And Nova launcher (like most launchers if not all of them) only displays the home screen and the app drawer, anything other than that (like recents, notifications and lock screen) is controlled by the android system not Nova launcher.
Note:The lock screen icons can be changed by a lock screen app but not Nova launcher.