.navbar {
  background: linear-gradient(-135deg, #72efe4, #9941D8);
}

navbarを目立たせても、あまり意味がないことがわかった

.navbar {
   background: linear-gradient( to bottom,
     red,
     orange,
     yellow,
     green,
     aqua,
     blue,
     purple,
     red                                      
   ) center  0% / auto 1400% ; /* グラデーション */
   animation: navbar 560s linear infinite;
   animation-direction: reverse;
 }
 @keyframes navbar {
   to { background-position-y: 1400%; }
 } 