<h1>Hello world!</h1>

:root {
   --background: white;
   --textColor: #000;
}

@media (prefers-color-scheme: dark) {
   :root {
      --background: #111;
      --textColor: #fafafa;
   }
}

body {
   display: flex;
   align-items: center;
   justify-content: center;
   text-align: center;
   height: 100%;
   min-height: 100vh;
   background-color: var(--background);
   color: var(--textColor);
}

h1 {
   font-size: 5rem;
}