Probably the easiest way would be to drill into the inside edges of the acrylic so led can go in. Then tie it to the +5v line from usb with the approritate resistor. It will give a cool diffused glow effect.
Adding like backlight will be quite a lot more work, and the pcb is not set up for it so quite a lot of wiring would be under the plate that way. You would have to desolder everything, insert the led into switch, bend the legs so they do not interfere and create your own wire matrix for the led, and so on. It's possible just a lot of effort.