I know that it’s possible to expand Scrivener’s selection of icons beyond the built-in choices, but I’m not quite sure how it’s done? I know where to get icons – at least Mac-format .icns files – and I know where the “Manage Icons…” menu choice is, and I’ve see the window that it brings up. But I can’t quite figure out how the icon management system works, or what I need to do to get the icons into it and showing up on the menu.
In summary, though, just hit the “+” button in the Manage Icons… panel. This will bring up the standard Finder Open panel - just browse to and select the icons you want to import.
There are two places you can import to: “Project Package” or “Application Support”. Icons imported to the former will be stored inside the project itself, and won’t be available to other projects. Icons imported to the latter will be available to all projects on your user account, but won’t show up if you transfer the project to another account or computer (unless you install the same icons on that machine, too).