You could probably change the stylesheet based on a incoming MQTT message. But I personally don’t think it’s worth the hassle. Dimming the light by adjusting colors on a webpage won’t conserve energy (on non-oled display’s) or prolong the lifetime of the device. Unless you are bothered by the light? There is an dark theme available inside node-red.
When I’m not around I turn the homescreen off with the help of homeyduino and a wire soldered to the magnetic smart cover sensor. I’ve also made a less intrusive solution by using an ESP32 to emulate a Bluetooth keyboard to lock and unlock the ipad with Homey:
Sadly this wont work with my old iPad2. It needs to support Bluetooth BLE to work.
