Would this be possible? I think it's kinda easy to implement.

I've created a wishlist because I couldn't find any: Night Theme GOG Site Wishlist.

For now, you can try the Dark Reader extension. Here's a preview of what it looks like.

Amazing. Thanks for the recommendation.
-edit:
And it's open source, even better.

Dark Reader is pretty heavy though and might significantly affect performance in some cases.

The current "Witcher Anniversary" theme is not bad to be honest... I would not mind if they kept it as default.

After some testing I can confirm this, unfortunately.
Looks like the best night theme right now though. Maybe their algorithm is processing way too many things at the same time. Wish they had a 'lite' version without all those configurations. Yes. That's what I thought.

Dark Reader also doesn't work on GOG because GOG's code is so old that it predates the common use of CSS sheets, and is in fact mostly a nightmare slush pile of outdated javascript.

I'd be down for a dark theme.

I'm currently using Dark Background and Light Text for Firefox.
It works and looks tons better than literally staring into a whitespace lightbulb but it's a bit janky with GoG (some buttons/selectable elements looks a bit odd), probably due to the aforementioned js 'issue'.
Doesn't seem to affect functionality though.

Mind it breaks the gog.com/account page (all dark themes I tested do), making it pretty hard to read the first files' info (or everything if your game doesn't have a lot of files) when you open the dropdown menu of your games' info.

Examples (2 images): https://imgur.com/a/pXg9ink