I don't know so much about fluxbox theming so I can't speak from my own experience here.
But, to me, your systax doesn't seem ok if what you want to use is a bitmap font (that's what you were after, weren't you?).
Fluxbox, like some other wm's, can handle fonts in many ways. For bitmap fonts the fontname should be something like what xfontsel says shows. You can copy/paste from it (that's what the "select" button in xfontsel is for). For example, go to "fmly", choose "terminus", click "select", then middle click in your text editor and it should paste the font name there for you:
You can select many more fields, for example, if you choose fmly terminus and pxlsz 12 you get this when pasting:
This is what you should be using, probably.
The other syntax is for xft, and you'd then be using scalable fonts which would make me think why did you want to fix the bitmap fonts issue to start with.