How to create dark mode? (CSS)

Not asking about the implementation of JavaScript.

Just a heads up, if you are planning to create a dark mode for your application, you no longer need to use JavaScript, you can simply do it by @media query.

For example

@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; }
}

I am want to know...

• How to create dark mode with different dark colors?
• How to select the colors and keep the contrast?
• Is there any specific color palette for dark theme?