#3F747E

CSS

:root {
  --mysterious-blue: #3f747e;
  --mysterious-blue-rgb: 63 116 126;
  --mysterious-blue-oklch: oklch(0.526 0.059 211.1);
}

.element {
  color: var(--mysterious-blue);
  background-color: hsl(190 33% 37%);
  border-color: oklch(0.526 0.059 211.1);
}

Tailwind CSS v4

@theme {
  --color-mysterious-blue: #3f747e;
  --color-mysterious-blue-50: #e3faff;
  --color-mysterious-blue-100: #d3edf4;
  --color-mysterious-blue-200: #b9dbe3;
  --color-mysterious-blue-300: #96c0c9;
  --color-mysterious-blue-400: #75a8b2;
  --color-mysterious-blue-500: #55919c;
  --color-mysterious-blue-600: #497b84;
  --color-mysterious-blue-700: #3a626a;
  --color-mysterious-blue-800: #29484d;
  --color-mysterious-blue-900: #1b3236;
  --color-mysterious-blue-950: #0b1e21;
}

SCSS

$mysterious-blue: #3f747e;
$mysterious-blue-rgb: 63, 116, 126;
$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);
}