#303D5B

CSS

:root {
  --mediaeval-blue: #303d5b;
  --mediaeval-blue-rgb: 48 61 91;
  --mediaeval-blue-oklch: oklch(0.363 0.055 265.8);
}

.element {
  color: var(--mediaeval-blue);
  background-color: hsl(222 31% 27%);
  border-color: oklch(0.363 0.055 265.8);
}

Tailwind CSS v4

@theme {
  --color-mediaeval-blue: #303d5b;
  --color-mediaeval-blue-50: #eef5ff;
  --color-mediaeval-blue-100: #e0e8fb;
  --color-mediaeval-blue-200: #cad4ed;
  --color-mediaeval-blue-300: #abb7d4;
  --color-mediaeval-blue-400: #8f9ec0;
  --color-mediaeval-blue-500: #7586ab;
  --color-mediaeval-blue-600: #637191;
  --color-mediaeval-blue-700: #4f5b74;
  --color-mediaeval-blue-800: #394256;
  --color-mediaeval-blue-900: #272e3c;
  --color-mediaeval-blue-950: #151b26;
}

SCSS

$mediaeval-blue: #303d5b;
$mediaeval-blue-rgb: 48, 61, 91;
$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);
}