Dimming can be accomplished one of two ways... 1.) It can be done thru a low-pass filter via PWM on the Rext current resistors. 2.) It can be done by modulating each LED's on-time by time slicing in the firmware. As each LED is only turned on or off by a "1" or a "0", you need to only leave the "1" on for each LED for a millisecond period of time. I have done this approach in the past and have the experience to write the firmware on a PIC>>.
Bill