The package sys-power/acpilight provides an xbacklight command which is compatible with (and extends) x11-apps/xbacklight but uses the system's ACPI to control the display. Please consider adding it as an alternative in the dependencies.
+1 :) " * Error: The above package list contains packages which cannot be * installed at the same time on the same system. (x11-apps/xbacklight-1.2.1-r1:0/0::gentoo, ebuild scheduled for merge) pulled in by x11-apps/xbacklight required by (x11-base/xorg-x11-7.4-r2:0/0::gentoo, installed) (sys-power/acpilight-1.0:0/0::gentoo, installed) pulled in by sys-power/acpilight required by @selected" Thanks!
(In reply to Brian Evans from comment #0) > [...] > Please consider adding it as an alternative in the dependencies. See https://github.com/gentoo/gentoo/pull/5002/commits/f3f65280f4cd2f3e12bc59f40560d666d278cc8c
commit 1009be81becb469ad7408544c67173db79312aca Author: charIes17 <charles17@arcor.de> Date: Mon Jul 10 08:46:59 2017 x11-wm/lumina: Bug 617838 and more Fix for https://bugs.gentoo.org/617838 Fix for https://bugs.gentoo.org/616590 Switch HOMEPAGE to https:// Introduce desktop-utils USE flag for Lumina's desktop utilities Use ${PV/_/-} for Lumina's patchsets Patch 1.2.0-desktop-files.patch Package-Manager: Portage-2.3.6, Repoman-2.3.1 Closes: https://github.com/gentoo/gentoo/pull/5002