You can check the history of your vehicle at http://www.etis.ford.com/vehicle/softwareUpgrades.do -> Summary -> History (registration as an independent operator is mandatory - and free). I see log entries in my vehicle
12-10-17 10:36
xxxxxx
G1582102
de
DE
Finished
Ford VCM II
Persisted Framework
12-10-17 09:34
xxxxxx
G1582102
de
DE
Starting
Ford VCM II
Persisted Framework
This perfectly matches up with when I had the vehicle serviced. This enables you to at least cross-check the claim that anything happened at all.
Unfortunately, I am unable to reliably interpret the information in the "Changes and Upgrades" at the above-mentioned URL. I can only speculate that Ford internally tracks the version information per vehicle (VIN) and thus only displays a link to "Software Updates" only if there are in fact updates to the software. For my vehicle, no updates to the BCM are flagged, but instead I see updates available for other modules, e.g.
Software Updates - Audio Front Control Module (ACM)
Module Programming
This application will update the Audio Control Module software to the latest level.
Prior to commencing programming it is important that the vehicle battery is connected to a suitable battery support unit.
20170320-G1820905-6
Perhaps, for your vehicle, this lists an update to the BCM being (still) available? This would at least provide hope that with an updated BCM the dark mode setting could be applied.