rpk is: Rpk extension is related to RadLight Media Player Skin. You have chance to open rpk files with RadLight Media Player software.
It is a theme file which created for a video player called RadLight. To install this format, you should download various themes from the radlight web page and put them into program folder. To get detailed information and check the themes, please check the following link for radlight.
Rpk File format viewers or openers
RadLight Media Player
Rpk File related links.