Putting light behind the keys is dificult, but not impossible (EL wire, LEDs, EL panel if you really want it as bright as possible), but all these will only illuminate the EDGES of the keys. Custom partially transparent keys would have to be created (likely at great expense unless they were made in bulk, and even then it wouldn't be cheap) in order to have the characters on the keys illuminate.
One idea that has been put forward is to painstakingly paint over the characters in UV fluorescent paint, coat this in a durable (and, importantly, UV transparent) topcoat to prevent the paint running off, then illuminating the whole top of the keyboard with UV LEDs. This would cause the keyboard characters to glow fairly vividly, but would run the risk of rubbing off through use, and is difficult to apply without a steady hand and a lot of spare time. Plus finding a topcoat may be impossible: most protective topcoats are designed to block UV in order to prevent yellowing.