The package sparky-backlight uses an icon called `preferences-desktop-display-color` for the tray. It isn't on my system in any installed icon packs which are Adwaita, GNOME, HighConstrast and Tango. With the exception of Tango they do all have `preferences-desktop-display`.
IMO the icon without `-color` is better anyway: (https://upload.wikimedia.org/wikipedia/commons/thumb/7/75/Breezeicons-apps-32-preferences-desktop-display.svg/32px-Breezeicons-apps-32-preferences-desktop-display.svg.png). Vs `-color` variant: (https://upload.wikimedia.org/wikipedia/commons/f/ff/Breezeicons-apps-32-preferences-desktop-display-color.svg). This little tool doesn't modify the color of the screen so that's not the right icon.
Not sure best way to resolve. Include a fallback icon to use if theme doesn't have one?
I am not sure if there is any generic/default icon set that can be relied upon.
Gnome Adwaita has this one that might be a good candidate: `video-display.svg`: (https://gitlab.gnome.org/GNOME/adwaita-icon-theme/-/raw/master/Adwaita/scalable/devices/video-display.svg)
https://wiki.sparkylinux.org/doku.php/sparky_backlight
https://github.com/sparkylinux/sparky-backlight/blob/master/bin/sparky-backlight