As I stated in my previous answer, you may use transluscent materials, it should do the trick if you still need some help on the matter.
such as on this powerbank with flashlight, the transluscent material is the one around the LED/reflective cone, obvisouly it is not the perfect plastic but you still can work around to make it look like it.
here is what happen when you turn the light on
here is how it looks like for pure back light (LED through a plain plastic surface)
with the lights on, this one is not that obvious, but again you can still increase the strengh of the light source, or make the material more transluscent, but in all case you must find the right balance to not lose the material property or having too bright light ( it all depend of your product and what you need to show underneath)