There's a dark theme that site administrators can switch to, but an interesting option would be to allow browsers to use the system setting to do this. That can be done with a @media (prefers-color-scheme: dark) media query, and an added option to the theme settings.
Needs review
1.7
Code
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It's likely this issue predates Contrib.social: some issue and comment data are missing.
Slight change in terminology, changing "Use browser setting" to "Use system setting" since it's (usually) the OS that sets the theme, not (normally) the browser.