#0C172D

CSS

:root {
  --sombre-night: #0c172d;
  --sombre-night-rgb: 12 23 45;
  --sombre-night-oklch: oklch(0.208 0.047 262.9);
}

.element {
  color: var(--sombre-night);
  background-color: hsl(220 58% 11%);
  border-color: oklch(0.208 0.047 262.9);
}

Tailwind CSS v4

@theme {
  --color-sombre-night: #0c172d;
  --color-sombre-night-50: #eff5ff;
  --color-sombre-night-100: #e0e8f8;
  --color-sombre-night-200: #cad4e9;
  --color-sombre-night-300: #acb8d0;
  --color-sombre-night-400: #909fbb;
  --color-sombre-night-500: #7687a6;
  --color-sombre-night-600: #64728c;
  --color-sombre-night-700: #505b70;
  --color-sombre-night-800: #394353;
  --color-sombre-night-900: #272e3a;
  --color-sombre-night-950: #151b24;
}

SCSS

$sombre-night: #0c172d;
$sombre-night-rgb: 12, 23, 45;
$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);
}