• Install the extension from the Chrome Web Store:
  https://chrome.google.com/webstore/detail/chrome-devtools-dark-them/bfcohnmjbpeilfcijkjkeggbdmehhnbk

• Enable DevTools Experiments in Chrome (http://islegend.com/development/how-to-enable-devtools-experiments-within-google-chrome/)

• Go to DevTools Settings -> Experiments -> Enable "Allow Custom UI themes".

• Go to DevTools Settings -> Preferences and select Theme: "Dark", which enables Chrome's own Dark Theme. This theme builds on top of that.

To customize this theme you can edit the CSS (SASS) by yourself:

• run npm install. Make sure you have at least node.js v4 installed.
• Install Gulp globally if you haven't done that yet: npm install --global gulp-cli
• modify the .scss under src/ to your preference
• compile the .scss to dist/theme.css by running gulp generate.css
• In dist/devtools.js, comment out line 9: var theStyle = ':host-context(.platform-mac) .mo [...]. Now the theme.css file gets loaded dynamically.
• Load the extension locally in Chrome: go to chrome://extensions/, enable 'Developer Mode', click 'Load unpacked extension' and load it (select the dist/ folder).
• To style the DevTools, you can actually inspect the DevTools window itself! Make sure the DevTools are undocked, e.g. they are in a seperate window, then press Cmd+Option+I and another DevTools window opens.

The CSS for this theme is based on the DevTools Author themes:
https://github.com/micjamking/devtools-author