I have two screenshots below of the same Medium blogpost. One in light mode and one in dark mode, using the dark reader extension.
Dark Mode
Light Mode
I think it's a transparent div that Medium uses for some reason, and it gets colored black by Dark Reader. Medium, why do you need this div?!?!
You are using an experimental theme 'GPU static' (located under the 'more' tab) that is setting the background colour of the div on the left to background-color: rgb(16, 20, 23) !important;
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The div element in question (the 'Publishous' section on the left) uses a fixed position (and is full page width on the parent element and positioned within that) so once the background is set it covers the text (as fixed position elements are placed on top of relative positioned elements).
Change away from the 'static GPU' setting and it will work fine.