Here is a dark theme for Fluxbox I created recently.
To install the theme unzip the theme archive below to .fluxbox/styles/ folder structure.
If you choose to use /usr/share/fluxbox/styles then you must point the location of the wallpaper manually by using feh or nitrogen (or something else). The theme is GPL
Further notes: In the case of Fluxbox it is recommended to install feh.
Wallpaper is also included in the theme archive.