session.imageDither: True?? (не специалист по fluxbox), но если там нет такого ключа, то и никак.
man fluxbox
session.imageDither:
True or False to, respectively, enable or disable dithering
of images. Only necessary on systems with small colour depths (8bpp or less)
session.colorsPerChannel: <integer>
This tells fluxbox how many colors to take from the X server on
pseudo-color displays. A channel would be red, green, or blue. fluxbox
will allocate this variable ^ 3 and make them always available. Value must
be between 2-6. When you run fluxbox on an 8bpp display, you must set this
resource to 4. Default: 4
