Unfortunately, Nabble can be slow to update its Help pages. I think you may have been looking at old information. The option you need is at:
Options > Application > Change appearance
You only see that menu sequence when logged in as administrator and on the root screen of your forum. Once selected the Nabble screen changes to show a black customisation menu bar. There is a CSS button that opens a dialogue where you may insert your custom CSS.
Volunteer Helper - but recommending that users move off the platform!
Once the admin for GregHelp now deleted.