For the record, here's the quick reduced testcase that I spun up; this one successfully honors `prefers-color-scheme:dark` inside of the iframe (as can be seen by the lime background loading properly), so it doesn't actually trigger the bug.
Bug 1908060 Comment 4 Edit History
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For the record, here's the quick reduced testcase that I spun up; this one successfully honors `prefers-color-scheme:dark` inside of the iframe (as can be seen by the lime background rendering properly), so it doesn't actually trigger the bug.