Browser update might do it, or explicitly clearing your browser history.
If you have an OS level darkmode setting that goes follows certain patterns, that could also explain changes. e.g. you have your OS set to be dark at night, then SN will use that setting.
Strange though. A couple of times I find the SN dark mode to be reset to light. Not doing updates. My OSX is always in dark theme. It seems buggy to me.
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Could be a bug. What browser do you use?
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Chrome on OSX
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I'll look into it
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Just woke up here in CEST. It reset again to light mode.
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And again! Bug is real.
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This hasn't been fixed yet
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