First, you need to know how a dimmer switch work so you can understand the principle how to control an AC voltage.
Here a link : How Dimmer Switches Work | HowStuffWorks
And go to NI$HANT tread. My design use this principle. Bear in mind it is only one device being control. To control multitude devices is no so easy for me at this time. It is going to be "tricky". It is the sychronization is going to be "tricky" part to control different AC devices at differents "dim" levels.