Dark mode is a reading preference, not a decoration. People turn it on at night, in a dim room, or because light text on a dark page is simply easier on their eyes. The page still has to be readable.
Do not use pure black and pure white
Pure white text on pure black looks harsh, and thin strokes can glow. A near-black background with off-white text is calmer and still has strong contrast. Check both modes the way a reader will: a paragraph, not a single word.
Keep the same hierarchy
Headings, body text and muted notes should stay distinct. If every line becomes the same pale grey, the scan that works in light mode disappears. Body text should stay the easiest thing to read.
Test the article, not the chrome
Nav bars and buttons get the attention. The long text in the middle is what people stare at. Read a full section in both modes before you call the theme done. If you squint or lean in, the contrast is not there yet.