This was an experiment in using rear-mounted LEDs, shining through drill holes.

Not that anyone would notice but I used curves traces to represent smoke.

Since I designed it to have square number of LEDs and I only ever expected to use a repeating pattern I was able to drive the LEDs using a single 8 bit shift register with 4 LEDs in parallel on each channel. The global brightness is set by PWM on the shift register’s output enable pin.