Define every colour once

How to do it

Put tokens such as --paper: light-dark(#fff, #14172e) on :root. A theme switch sets data-theme, which sets color-scheme. The colour scheme rule covers the meta tag and the first paint.

Example

:root {
  color-scheme: light dark;
  --paper: light-dark(#fff, #14172e);
  --ink: light-dark(#1b1f3b, #e8eaf6);
}
:root[data-theme="light"] { color-scheme: light; }
:root[data-theme="dark"] { color-scheme: dark; }
body { background: var(--paper); color: var(--ink); }

How to check

Search the stylesheet. No colour values appear outside :root, apart from black stops in masks, which set transparency rather than colour.