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As if it wasn't bad enough that they want me to use a random internet service to add a keyboard to a usb wifi receiver, they have the balls to put this for Firefox users. I clicked out of pure curiosity, as I'm not even remotely interested in involving a corporate internet service in getting my keyboard connected to my computer. This is the message you get now on Logi Options software if you have a Unifying Receiver: 
For the curious: https://logiwebconnect.com
EDIT: some people on the thread have brought up that the error message being displayed for Firefox users is due to the WebUSB API not being implemented by Firefox due to security concerns. This still does not justify having to use a web app to plug peripherals to a PC.
I bought a Logitech mechanical keyboard which happened to be RGB. I did not specifically want an RGB keyboard but it was the one mechanical one that was on sale at the time, and all the cheap(er) mechanical keyboards seem to be the gaming ones these days.
Yeah, so apparently there is no way to natively control or even turn off the lighting from Linux and it always defaults to the most obnoxious scrolling rainbow light show. My home office which I use for working night shifts remotely looks like a goddamn rave, really easy on the eyes when I'm sleep deprived as fuck at 3 AM. Is defaulting to a soft white backlight or something that much to ask for?
Did you try openrgb? Just curious!
https://openrgb-wiki.readthedocs.io/en/latest/Logitech-Keyboards/
Works with my Logitech non-rgb white only keyboard at least. Setup a profile and you can easily run that at boot.
Other than that, your keyboard may have a button for the lighting. I have to press the special button and 0 to get the always on light - the default is this annoying wave animation. So I can set it without software even, though OpenRGB is just more convenient still.