#51789F

CSS

:root {
  --into-the-blue: #51789f;
  --into-the-blue-rgb: 81 120 159;
  --into-the-blue-oklch: oklch(0.56 0.075 249.3);
}

.element {
  color: var(--into-the-blue);
  background-color: hsl(210 33% 47%);
  border-color: oklch(0.56 0.075 249.3);
}

Tailwind CSS v4

@theme {
  --color-into-the-blue: #51789f;
  --color-into-the-blue-50: #e6f7ff;
  --color-into-the-blue-100: #d6eaff;
  --color-into-the-blue-200: #bdd7f4;
  --color-into-the-blue-300: #9cbbdd;
  --color-into-the-blue-400: #7ca2ca;
  --color-into-the-blue-500: #5e8ab6;
  --color-into-the-blue-600: #50759a;
  --color-into-the-blue-700: #405e7b;
  --color-into-the-blue-800: #2d455b;
  --color-into-the-blue-900: #1e3040;
  --color-into-the-blue-950: #0e1c29;
}

SCSS

$into-the-blue: #51789f;
$into-the-blue-rgb: 81, 120, 159;
$into-the-blue-light: mix(#fff, $into-the-blue, 20%);
$into-the-blue-dark: mix(#000, $into-the-blue, 20%);

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