David Kimura PRO said over 2 years ago on Dark theme :
  This dark mode feature is live. You will need to actively enable it under your profile settings. I did this as to not affect existing functionality of the site as some may prefer to have light mode enabled regardless of the OS setting. You can access the profile setting under https://www.driftingruby.com/users/edit

Not all places may be complete, but I did a quick look through. If any areas are difficult to read or any suggestion of color theming, I'm willing to make some adjustments.