#0D172B

CSS

:root {
  --sombre-night: #0d172b;
  --sombre-night-rgb: 13 23 43;
  --sombre-night-oklch: oklch(0.207 0.043 263.1);
}

.element {
  color: var(--sombre-night);
  background-color: hsl(220 54% 11%);
  border-color: oklch(0.207 0.043 263.1);
}

Tailwind CSS v4

@theme {
  --color-sombre-night: #0d172b;
  --color-sombre-night-50: #eff5ff;
  --color-sombre-night-100: #e1e8f7;
  --color-sombre-night-200: #cbd4e7;
  --color-sombre-night-300: #adb8ce;
  --color-sombre-night-400: #919fb8;
  --color-sombre-night-500: #7787a3;
  --color-sombre-night-600: #65728a;
  --color-sombre-night-700: #515b6e;
  --color-sombre-night-800: #3a4351;
  --color-sombre-night-900: #282e39;
  --color-sombre-night-950: #161b23;
}

SCSS

$sombre-night: #0d172b;
$sombre-night-rgb: 13, 23, 43;
$sombre-night-light: mix(#fff, $sombre-night, 20%);
$sombre-night-dark: mix(#000, $sombre-night, 20%);

.element {
  color: $sombre-night;
  box-shadow: 0 0 0 2px rgba($sombre-night, 0.35);
}