Unfortunately it seems like Custom icons loader is broken since HA 0.110.
See https://github.com/Armaell/home-assistant-custom-icons-loader/issues/5.
For anyone stumbling across this in 2021, hass-hue-icons is a set of colourizable svg icons that extends home assistant to model the hue product range.
See HACS or GitHub - arallsopp/hass-hue-icons: Additional vector icons for home assistant to model Philips Hue bulbs
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