I am using this chrome extension.

https://chrome.google.com/webstore/detail/dark-reader/eimadpbcbfnmbkopoojfekhnkhdbieeh

It is called "Dark Reader". Its file size is less than 1 MB. Follow the link above and you will see many 5 stars rating.

It is very easy to use and customizes. No need to messing with CSS codes. Just try it.

You can always uninstall if you do not like.

Hope this help.