I also have a similar issue.
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Navigation bar text colors are dark, with the background dark, making text hard to see.
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The reply's background is white, although the text boxes and the preview has a dark background.
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Icons are gray in a red background (also hard to see)
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Quotes have a white header
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"Last visit" has a white background
Very strange, and Discourse is gonna fix it.
Edit: By the way, I'm not using any extensions nor userscripts at all.