:root { color-scheme: dark }

html, body { height: 100% }

body {
  margin: 0;
  background: #000;
}

main {
  /* Definite height plus border-box padding, so the logo's max-width and
     max-height below resolve against a known box rather than a circular
     content-sized one. */
  height: 100%;
  box-sizing: border-box;
  padding: clamp(1rem, 4vmin, 3rem);
  display: flex;
  align-items: center;
  justify-content: center;
}

.logo {
  display: block;
  /* Half of the available box, so the logo sits as a mark on the page rather
     than filling it. Whichever axis runs out first binds, keeping the aspect
     ratio; on a desktop that is the height, on a phone the width. */
  width: auto;
  height: auto;
  max-width: 50%;
  max-height: 50%;
}