I feel like I need to post this whenever retr0bright is brought up just so people remember:
Remember, retr0brighting changes the chemical composition of the plastic, making it weaker than it was before.
Do not retr0bright if the plastic is already brittle and do not retr0bright if you are looking to preserve something and make it maintain the colors. Retr0brighting brittle plastic may cause it to break or break more readily.
The Danish National Museum did a study on retr0bright and does not recommend it for the purposes of preservation due to the weakened state of the plastic and the damage it causes to the plastic. Also, remember that the plastic changes colors due bromine in it reacting to air, not exposure to UV light. If you have an old keyboard or something else made of plastic that you're looking to slow or stop the yellowing in, seal it with some kind of clear finish. That should help retard the reaction that causes it to yellow.