#5A7B8A

CSS

:root {
  --night-owl: #5a7b8a;
  --night-owl-rgb: 90 123 138;
  --night-owl-oklch: oklch(0.563 0.044 227.7);
}

.element {
  color: var(--night-owl);
  background-color: hsl(199 21% 45%);
  border-color: oklch(0.563 0.044 227.7);
}

Tailwind CSS v4

@theme {
  --color-night-owl: #5a7b8a;
  --color-night-owl-50: #e9f8ff;
  --color-night-owl-100: #daebf4;
  --color-night-owl-200: #c2d8e4;
  --color-night-owl-300: #a1bcca;
  --color-night-owl-400: #84a4b3;
  --color-night-owl-500: #678c9d;
  --color-night-owl-600: #587785;
  --color-night-owl-700: #465f6b;
  --color-night-owl-800: #32464e;
  --color-night-owl-900: #223036;
  --color-night-owl-950: #111c21;
}

SCSS

$night-owl: #5a7b8a;
$night-owl-rgb: 90, 123, 138;
$night-owl-light: mix(#fff, $night-owl, 20%);
$night-owl-dark: mix(#000, $night-owl, 20%);

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