To add a new Room Effect, all you need to do is add a new entry to the RoomSettings.RoomEffect.Type enum. Once you do that, your Room Effect will be visible and usable with Dev Tools.
However, it obviously won't do anything yet! You can utilize the Room Effect to do things anywhere that you have a reference to the room object. To read the Room Effect, you use:
If the effect has not been selected as an active effect in the room, this will return 0.0. Otherwise, it will return a value between 0.0 and 1.0, depending on where the slider for the Room Effect was set in Dev Tools. Then it's up to you what you want to do with this value, the sky is the limit...