The *navbox card is code-governed here: https://github.com/wagn/wagn/blob/master/mods/standard/sets/self/navbox.rb
Looks like there's an ugly hack in place that would let you add a placeholder by creating a *navbox+*placeholder card. I suspect we'll want to create a better method before this becomes a supported practice (ie, I wouldn't document that trick in its current state). But for now it should work.
So, if I make a card called that, how do I use it? Make it a Basic card and have the contents be the word that I'd want to appear in the navbox? How do I style the navbox? I see on this site you have a .navbox class. Does it only work if you call it that? I tried putting some style settings under a different name, but they didn't do anything.
I tried it on one of my test Wagns. I'd make it a Phrase type so you don't get any extra tags in it.
As for the style settings, maybe your selector isn't matching. I use the inspector to figure out such things. Remember that you have to have a more specific selector to override another one.