It’s all done with CSS animations [1].

The background specified in interconnected-2020.css [2] lines 8–9:

  background: #cdecff; /* lightest-blue */
  animation: changingbg 600s infinite;

Followed by the animation specified in lines 12–35:

  /* Calculated using
     https://www.joshwcomeau.com/gradient-generator/
     in LRGB mode with precision=8 and linear easing
     Colors: #CDECFF #FFCDEC #FEECCD #9EEBCF #CDECFF
  */
  @keyframes changingbg {
    from {
      background: hsl(203deg 100% 90%);
    }
    8% { background: hsl(233deg 68% 92%); }
    17% { background: hsl(293deg 52% 90%); }
    25% { background: hsl(323deg 100% 90%); }
    33% { background: hsl(344deg 98% 92%); }
    42% { background: hsl(16deg 97% 92%); }
    50% { background: hsl(38deg 96% 90%); }
    58% { background: hsl(78deg 44% 87%); }
    67% { background: hsl(137deg 50% 84%); }
    75% { background: hsl(158deg 66% 77%); }
    83% { background: hsl(185deg 62% 84%); }
    92% { background: hsl(191deg 87% 84%); }
    to {
      background: hsl(203deg 100% 90%);
    }
  }

[1]: https://developer.mozilla.org/en-US/docs/Web/CSS/CSS_Animati...

[2]: https://interconnected.org/home/static/styles/interconnected...