Dark mode is six numbers, not a second design

The usual approach is a second block of colour variables under a media query. It works on the day you write it, and then someone adds a colour to the light theme, forgets the dark one, and a card goes white on white for four months before anyone reports it.

The fix is structural. Define the palette as a composition — lightness, chroma, hue — and let the scheme swap only the lightness values. Six numbers change. Every colour in the theme is downstream of them, so nothing can be added to one scheme and missed in the other, because there is only one set of colours.

Two things still need judgement

Accent lightness has to rise in dark mode, because a mid-lightness accent that reads confidently on white reads muddy on near-black. And ambient washes need more opacity, not less — the same gradient that is a suggestion on paper is invisible on charcoal.