After downloading the game using the Firmament GOG game code, I then clicked on and received the Firmament Backer Content GOG (DLC) key. The Cyan Kickstarter update #57 says after getting the DLC key you can access a "Customization" menu from within the in-game menu. However, I do not see any "Customization" menu options. What am I missing?

The Customisation menu is only available after starting the game.
If it does not appear, check GOG Galaxy to confirm you have the "Firmament - Kickstarter Backer Content" item under "Extras", "Offline Backup Installers".

If it does not appear there, you have not redeemed the DLC key correctly.

Thanks for your reply. II had not separately redeemed the DLC key; thanks again.

Hi Goldgesicht,
This is the same as I experience. I have been in contact with support, very helpful person Aleksandra trying to help;
As pr. her instruction you should, quote:
"Launch the GOG GALAXY app, select the game Firmament in your "Owned games", click the sliders icon next to the "Play" button > Manage installation > Configure > open the DLC tab and check if the DLC is displayed as available / installed."
unquote.
Speaking for me, I am not sure if I understand how this "backer content" is supposed to appear or function and feel maybe I am expecting too much.

Thank you. It was very helpful :-) To be honest, I don´t know ether what the content is, but I guess it is according to the tier.

Hello again :), I got a new mail with this explanation, quote:
"according to the Firmament's Kickstarter Campaing page, the "Backer content" DLC includes "three unique skins for your mechanical adjunct (including a special Myst skin!), uniquely skinned items and elements, AND special personalization options that allow customization of some narrative elements of the experience (example: Imagine a book on a shelf that has a title that you choose. Or a metal plate on a piece of equipment with a date that you decide)!" :)
Unquote.
So there it is, the whole bundle. And you will see your name somewhere in the credits at the end. Can’t say they went overboard, but it is what it is :)