I've managed to change the activity app icon in the menu, creating a new icon and calling it from a modified owncloud/apps/activity/lib/AppInfo/Application.php file.
Look, I've changed the Activity app's lightning icon for a cone,
This works, but is this the way?
I'm using a new Theme, it works perfectly, but I can't find a way of modifying the icons for default apps within my theme so it gets safe in case of updates, etc.
I'm using 9.1.2 stable version of ownCloud in Ubuntu Server 16.04