#51A4C7

CSS

:root {
  --mystic-blue: #51a4c7;
  --mystic-blue-rgb: 81 164 199;
  --mystic-blue-oklch: oklch(0.681 0.095 228.2);
}

.element {
  color: var(--mystic-blue);
  background-color: hsl(198 51% 55%);
  border-color: oklch(0.681 0.095 228.2);
}

Tailwind CSS v4

@theme {
  --color-mystic-blue: #51a4c7;
  --color-mystic-blue-50: #dbfbff;
  --color-mystic-blue-100: #c8efff;
  --color-mystic-blue-200: #abddf6;
  --color-mystic-blue-300: #83c1df;
  --color-mystic-blue-400: #5caacb;
  --color-mystic-blue-500: #2f92b8;
  --color-mystic-blue-600: #2a7b9b;
  --color-mystic-blue-700: #22637d;
  --color-mystic-blue-800: #15495c;
  --color-mystic-blue-900: #0d3340;
  --color-mystic-blue-950: #011e29;
}

SCSS

$mystic-blue: #51a4c7;
$mystic-blue-rgb: 81, 164, 199;
$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);
}