You can achieve it with Stylus overwriting the style of askubuntu.com with AskUbuntu-Dark-Theme

1. Install Stylus
2. Go to https://askubuntu.com/
3. Cick on Stylus icon and Write style for askubuntu.com
4. Paste style dark and Override style for: askubuntu.com
5. Save and ejoy!

More instructions: Usercss Wiki

You can achieve it with Stylus overwriting the style of askubuntu.com with AskUbuntu-Dark-Theme

1. Install Stylus
2. Go to https://askubuntu.com/
3. Cick on Stylus icon and Write style for askubuntu.com
4. Paste style dark and Override style for: askubuntu.com
5. Save and ejoy!

More instructions: Usercss Wiki

If you want a global dark mode use: UltimaDark

Note: if the extension break webpages like askubuntu.com use stylus to fix it and exclude the webpages on UltimaDark, enjoy :D