The game runs fine on Steam Deck. You can change to a higer resolution, which makes the text tinier and puts more content on screen, because it's a game from the late 1990ies. Don't enable 32-bit video mode! It did not work and it makes not much sense anyways for these kind of games. Many graphics cards and games at the time claimed to be 32-Bit capable, but there was no big difference. If you get stuck with an non working configuration on Steam Deck, choose software renderer from the startup options and change the options again in the game menu. Good luck and enjoy.

This bundle is said to include the Dark Consipracy extension, and indeed it does, but you'd have to dig a bit to find out how. One of the manual files in the game folder tells you that it is run from the same exe but needs the command line option "-mod DARKCONS". Since I have seen multiple launch options for other games but I don't know how to do that myself, Steam should add this to the out of the box experience.

I was not able to launch the game from the Steam Deck desktop, there it provides an additional launcher to launch the main game or the extension, but then the screen turn black and I don't know what to do. But I'm on Steam Deck for a reason, so I don't use the desktop unless necessary.