#3F808A

CSS

:root {
  --mysterious-blue: #3f808a;
  --mysterious-blue-rgb: 63 128 138;
  --mysterious-blue-oklch: oklch(0.561 0.068 208.6);
}

.element {
  color: var(--mysterious-blue);
  background-color: hsl(188 37% 39%);
  border-color: oklch(0.561 0.068 208.6);
}

Tailwind CSS v4

@theme {
  --color-mysterious-blue: #3f808a;
  --color-mysterious-blue-50: #e0fbff;
  --color-mysterious-blue-100: #cfeff5;
  --color-mysterious-blue-200: #b4dde5;
  --color-mysterious-blue-300: #8fc1cb;
  --color-mysterious-blue-400: #6daab4;
  --color-mysterious-blue-500: #4a939e;
  --color-mysterious-blue-600: #3f7c86;
  --color-mysterious-blue-700: #32636b;
  --color-mysterious-blue-800: #23494e;
  --color-mysterious-blue-900: #173336;
  --color-mysterious-blue-950: #081e21;
}

SCSS

$mysterious-blue: #3f808a;
$mysterious-blue-rgb: 63, 128, 138;
$mysterious-blue-light: mix(#fff, $mysterious-blue, 20%);
$mysterious-blue-dark: mix(#000, $mysterious-blue, 20%);

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