Yeah the driver supporting LEDs and exposing them should be installed. The exposed LEDs can be found in /sys/class/leds/<device>/multi_[index|intensity], See Linux kernel documentation for details: LED handling under Linux and Multicolor LED handling under Linux
I'm not familiar with KDE's new feature yet, but if it only supports sysfs LEDs then it won't control 99% of keyboards. Few RGB keyboards have drivers that expose this interface. Most RGB keyboards are controlled from userspace on their official software on Windows, and that's also what most Linux projects that control RGB devices including my OpenRGB project do. I wonder if it would be possible to write an OpenRGB plugin/script that exposes a virtual /sys/class/leds/openrgb device that KDE could talk to, then translate that into OpenRGB calls to set the color on all available devices. It doesn't sound too difficult.
Where did you see this information?
The ability for a screen to sync with something like an RGB backlight would be about the driver. Some RGB lights on keyboards don't even have an interface controller, meaning they just cycle whatever is programmed into the keyboard controller itself (Lenovo as an example). Are you sure you even have a controllable one?
It's in the announcement for Plasma 6.1, see https://kde.org/announcements/plasma/6/6.1.0/
To enable it, you need to use the Brightness & Colour widget. See also the merge request for this: https://invent.kde.org/plasma/plasma-workspace/-/merge_requests/4093
I guess the documentation is a bit lagging still! I don't know about a list of compatible keyboards, but I suppose you can just try it out to see if it works! 😁
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