html, body {
  margin: 0;
  height: 100%;
}

body {
  background-color: #16222c;   /* deep navy-slate tone */
  display: flex;               /* enable flexbox */
  align-items: center;         /* vertical centering */
  justify-content: center;     /* horizontal centering */
}

img.fit-height { 
  height: 100%;  /* key line */
  width:  auto;  /* keeps aspect ratio */
  display:block; 
}