After having worked on a dark theme for the forum (as I mentioned in another thread recently), here’s one for the main website.
Just like the other one, it requires a style manager addon (e.g. Stylus) for Firefox or Chromium/Chrome.
More info, installation link and screenshots: https://github.com/maxigaz/freecodecamp-dark-theme
You are aware that freeCodeCamp already has a built in night mode?
Do you mean the area where you type your code or the whole webpage? I’m only aware of the former.
Update: I’ve just found a button “Night Mode” in the account settings. Using Firefox 59.0.1, when I click on it, the colours change on that very page, but as soon as I navigate away (for example by clicking on “Update my email” or the freeCodeCamp logo in the corner), the colours reset to default. Also, it doesn’t seem to affect the sidebar when I click on “Map”, even if I stay on the settings page.