Function get_backlight 

pub async fn get_backlight(
    backend: &dyn HidBackend,
    route: &DeviceRoute,
) -> Result<BacklightState, WriteError>

Read the current backlight state of the keyboard on route.

FeatureUnsupported when the device does not expose HID++ 0x1982 — RGB keyboards (0x8070 / 0x8080) and every mouse fall in that group.