#51A4C8

CSS

:root {
  --mystic-blue: #51a4c8;
  --mystic-blue-rgb: 81 164 200;
  --mystic-blue-oklch: oklch(0.681 0.096 228.9);
}

.element {
  color: var(--mystic-blue);
  background-color: hsl(198 52% 55%);
  border-color: oklch(0.681 0.096 228.9);
}

Tailwind CSS v4

@theme {
  --color-mystic-blue: #51a4c8;
  --color-mystic-blue-50: #dafbff;
  --color-mystic-blue-100: #c8efff;
  --color-mystic-blue-200: #abddf6;
  --color-mystic-blue-300: #83c1e0;
  --color-mystic-blue-400: #5ca9cc;
  --color-mystic-blue-500: #2f92b9;
  --color-mystic-blue-600: #2a7b9c;
  --color-mystic-blue-700: #22637d;
  --color-mystic-blue-800: #15495c;
  --color-mystic-blue-900: #0d3240;
  --color-mystic-blue-950: #011e29;
}

SCSS

$mystic-blue: #51a4c8;
$mystic-blue-rgb: 81, 164, 200;
$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);
}