Is it permissible to participate in the Earth Hour event, in which lights and electronic devices are switched off for one hour to raise awareness of the danger of climate change, noting the participation of many Arab countries in it?

There is no harm in participating in this event because it is a custom, and the default ruling for customs is permissibility unless there is a specific prohibition from the Lawgiver. Therefore, if a custom is not proven to be forbidden, how can it be ruled as prohibited?