#21739C

CSS

:root {
  --chill-of-the-night: #21739c;
  --chill-of-the-night-rgb: 33 115 156;
  --chill-of-the-night-oklch: oklch(0.527 0.099 235.5);
}

.element {
  color: var(--chill-of-the-night);
  background-color: hsl(200 65% 37%);
  border-color: oklch(0.527 0.099 235.5);
}

Tailwind CSS v4

@theme {
  --color-chill-of-the-night: #21739c;
  --color-chill-of-the-night-50: #dcfaff;
  --color-chill-of-the-night-100: #caeeff;
  --color-chill-of-the-night-200: #addbfa;
  --color-chill-of-the-night-300: #86bfe4;
  --color-chill-of-the-night-400: #60a8d1;
  --color-chill-of-the-night-500: #3690bf;
  --color-chill-of-the-night-600: #307aa1;
  --color-chill-of-the-night-700: #266181;
  --color-chill-of-the-night-800: #18485f;
  --color-chill-of-the-night-900: #0f3243;
  --color-chill-of-the-night-950: #021e2b;
}

SCSS

$chill-of-the-night: #21739c;
$chill-of-the-night-rgb: 33, 115, 156;
$chill-of-the-night-light: mix(#fff, $chill-of-the-night, 20%);
$chill-of-the-night-dark: mix(#000, $chill-of-the-night, 20%);

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