I believe that many of the users love dark theme environment to work with. In this lockdown-quarantine period, I have tried to design dark theme extension for Codeforces.
Download the extension folder from the github link.
Go the extension management page of your browser (For chrome it is chrome://extensions)
Turn on the developer mode
Select 'Load unpacked' and browse and select the 'extension' folder which includes json and js files.
I am a beginner in this field. Please forgive me if it's a little buggy, but works well in my opinion. I request Codeforces team to design an official dark theme. Till then, kindy enjoy this one. Suggestions are welcomed. :)
Hope you'll find it helpful.