How does LiFi work when you turn off the lights?

If all power to a light is turned off then there is no LiFi. However, LiFi technology can be enabled to dim low enough that a room will appear dark and still transmit data. There is consistent performance between 10 and 90 percent illumination.

Currently, pureLiFi’s technology provide communications at light levels down to 60 lux. For comparison the British standard for minimum light level for reading is 400 lux.
There are also other options for using invisible parts of the light spectrum such as infra-red, which is currently already being used for sending information back to the lightbulb (uplink).