css variables

:root {
  --accent: #226997;
  --main: #333;
  --light: #666;
  --lighter: #f3f3f3;
  --border: #e6e6e6;
  --bg: #ffffff;
}

/* dark mode */
:root {
  --accent: #3493d1;
  --main: #f3f3f3;
  --light: #ececec;
  --lighter: #666;
  --border: #e6e6e6;
  --bg: #333333;
}

css & dark theme

demo

https://create-react-app.dev/docs/getting-started/#creating-an-app

prefers-color-scheme

https://developer.mozilla.org/en-US/docs/Web/CSS/@media/prefers-color-scheme


.day {
  background: #eee; color: black;
}
.night {
  background: #333; color: white;
}

@media (prefers-color-scheme: dark) {
  .day.dark-scheme   {
    background:  #333; color: white; 
  }
  .night.dark-scheme {
    background: black; color:  #ddd;
  }
}

@media (prefers-color-scheme: light) {
  .day.light-scheme   {
    background: white; color:  #555;
  }
  .night.light-scheme {
    background:  #eee; color: black;
  }
}

.day, .night {
  display: inline-block;
  padding: 1em;
  width: 7em;
  height: 2em;
  vertical-align: middle;
}

https://caniuse.com/#search=prefers-color-scheme