Two undocumented (meaning they are not in the fluxbox-style man page) are for setting the border color of focused and unfocused windows -
Unfortunately, the window boreder in fluxbox is not limited to the outside edges of the window; the border is also internal to the titlebar, tab, grip, and handle. So adding color to the focused window can get kind of ugly:
of course, a user can blend that into the rest of the fluxbox style so that the focused window would be - in this example - a matching gray palette while the unfocused window would be black.
A bit more interesting is going with BORDER as the default decor. In the ~/.fluxbox/init file go with:
# note that settings in the ~/.fluxbox/apps file override this
to provide more of a tiling aethetic -
Something to tinker with.