How are the LEDs in the bars wired?
Do you know the LED specifications?
Do you expect the individual LEDs in the bars to fade separately, or the whole bar to fade as one?
How do you want to control the fading patterns?
What else do you want the Arduino to do?
What power supply would you like to use?
Presumably you have an Arduino UNO?
If the Arduino needs to do nothing else, or whatever else it does is quite simple (or can be done while the LEDs are off), then all the PWM can be done easily in software (sketch).
Can we have a photo or two (in perfect focus and no more than 1024 by 768 resolution)?