I suspect that the cause of the issue is outlined in the “Default Dark Modes: full color invert” section of this post: Dark Mode Email: Your Ultimate How-to Guide - Litmus.
The Full Color Invert is the most invasive color scheme: It not only inverts the areas with light backgrounds, but impacts dark backgrounds as well.
So if you already designed your emails to have a dark theme, this scheme will ironically force them to become light . Unfortunately, this is currently the tactic used by some of the more popular email clients, such as Gmail app (iOS), Outlook 2021 (Windows), Office 365 (Windows), and Windows Mail.
That post’s example of the issue matches what you’re reporting:
Someone with access to Discourse’s Litmus account will need to test this. To reproduce the issue, try sending a digest email to Litmus from a Discourse site that has its default theme’s color pallet set to a Dark color scheme.
Assuming that triggers the issue, the easy fix would be to always use a Light color scheme for emails, regardless of the color palette that’s been assigned to the default theme on Discourse. There’s also an approach suggested in the “Gmail iOS & Outlook Windows-specific targeting” section of the post I linked to that could potentially work. It’s a bit of a hack though.
Whoever ends up looking into this might also look at the way the apply custom styles to digest site setting currently needs to be enabled to add dark mode support for email clients that actually do support dark mode. There’s nothing in the Discourse UI that makes it obvious that this setting is related to dark mode support.