Based on Solarized Dark Skin by Mark Osborne:
Inspired by Darcy Clarke's blog post
Ubuntu: ~/.config/google-chrome/Default/User\ StyleSheets/Custom.css
Windows: C:\Users\YourUsername\AppData\Local\Google\Chrome\User Data\Default\User StyleSheets\Custom.css
Mac: ~/Library/Application\ Support/Google/Chrome/Default/User\ StyleSheets/Custom.css
You can use the normal Source editor or Codemirror (in about:flags), both are suported.