Just to confirm, have you made any changes to the Discourse HTML email template that’s found at Admin / Customize / Email Style? Also, have you added any CSS to the CSS tab that is found on that page?
Also, is the apply custom styles to digest site setting enabled on your site? That setting is enabled by default.
Assuming that any customizations you have made are not causing the issue, my guess is that what’s happening is that the %{dark_mode_styles} placeholder that’s in the default Discourse email template is being replaced with the CSS that’s added by this method:
That method adds the following CSS to the template:
[dm='header'] {
background: #151515 !important;
}
That CSS is intended to cause the digest header to be dark when the email is viewed in a device that’s set to dark mode, but maybe some devices are ignoring the !important directive and displaying the inverse of the color.
I can’t reproduce the issue on my Android phone. I don’t have an iPhone to test it on. Maybe someone else can try viewing a summary email in dark mode on an iPhone?