#51A4C9

CSS

:root {
  --mystic-blue: #51a4c9;
  --mystic-blue-rgb: 81 164 201;
  --mystic-blue-oklch: oklch(0.682 0.097 229.6);
}

.element {
  color: var(--mystic-blue);
  background-color: hsl(199 53% 55%);
  border-color: oklch(0.682 0.097 229.6);
}

Tailwind CSS v4

@theme {
  --color-mystic-blue: #51a4c9;
  --color-mystic-blue-50: #dafbff;
  --color-mystic-blue-100: #c8eeff;
  --color-mystic-blue-200: #abdcf7;
  --color-mystic-blue-300: #83c1e1;
  --color-mystic-blue-400: #5ca9cd;
  --color-mystic-blue-500: #2f92ba;
  --color-mystic-blue-600: #2a7b9d;
  --color-mystic-blue-700: #22637e;
  --color-mystic-blue-800: #14495d;
  --color-mystic-blue-900: #0d3241;
  --color-mystic-blue-950: #011e29;
}

SCSS

$mystic-blue: #51a4c9;
$mystic-blue-rgb: 81, 164, 201;
$mystic-blue-light: mix(#fff, $mystic-blue, 20%);
$mystic-blue-dark: mix(#000, $mystic-blue, 20%);

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