#313A5C

CSS

:root {
  --mediaeval-blue: #313a5c;
  --mediaeval-blue-rgb: 49 58 92;
  --mediaeval-blue-oklch: oklch(0.357 0.06 271.8);
}

.element {
  color: var(--mediaeval-blue);
  background-color: hsl(227 30% 28%);
  border-color: oklch(0.357 0.06 271.8);
}

Tailwind CSS v4

@theme {
  --color-mediaeval-blue: #313a5c;
  --color-mediaeval-blue-50: #eff4ff;
  --color-mediaeval-blue-100: #e1e7fd;
  --color-mediaeval-blue-200: #ccd3ef;
  --color-mediaeval-blue-300: #adb6d7;
  --color-mediaeval-blue-400: #919dc3;
  --color-mediaeval-blue-500: #7884af;
  --color-mediaeval-blue-600: #657094;
  --color-mediaeval-blue-700: #515976;
  --color-mediaeval-blue-800: #3a4258;
  --color-mediaeval-blue-900: #282d3d;
  --color-mediaeval-blue-950: #161a27;
}

SCSS

$mediaeval-blue: #313a5c;
$mediaeval-blue-rgb: 49, 58, 92;
$mediaeval-blue-light: mix(#fff, $mediaeval-blue, 20%);
$mediaeval-blue-dark: mix(#000, $mediaeval-blue, 20%);

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