Normal 240VAC lighting is usually setup that the power starts at the switchboard, goes down the wall to the switch, a feed for the next room/light and the light feed go back up the wall and to the light. This hasn’t really changed in recent history with the exception of inline control devices that add some smarts directly to the light. Ebay has many RGB remote control LED lights now that pug straight into regular E27 fitting. The downside of these is, apart from most of the being IR, that the light switch must always be on for the device to work. LifX is similar but uses a bluetooth mesh network for its control, much easier to control centrally.
240VAC wiring is now overspeced in the LED lighting world, although it has proved to be extremity versatile. The major downside in converting existing 240VAC systems to LED lighting is the transformers, anyone who has done this conversion will know the annoying flicker from the cheap switch mode power supplies.
In powering a full 12VDC system though, here is the first major decision. Do you have a 12VDC power bus running around the roof and each light/circuit having a remotely controlled switch (many things could do this; relay, mosfet, arduino, i2c) or do you run each circuit/light back to a central place and control the light from a single source?
I decided to go with the second option of running back to a single source. The does mean more cabling but that cable would overall be less as running a power buss that is capable of handling a full load (that is every light on at the same time and a bit more) would be massive (0/1 AWG or welding cable).
Doing some quick tests showed that my 15W LED, that should be pulling 1.25A at 12CDC actually pull 0.6A for some reason, given total loads on a circuit I can very safely get away with running standard 1mm^2 twin core cable.