Enable Developer Tools Experiments
Navigate to chrome://flags in your address bar. Flags allow you to access more advanced browser configuration. Scroll down to "Enable Developer Tools experiments" and click Enable.
Enable custom UI themes
Open Developer Tools. Click the settings icon (the sprocket icon in the top right) and select "Experiments". Scroll down and check the box next to "Allow custom UI themes". Close Developer Tools.
Download a theme
There's an excellent dark theme called DevTools Theme: Zero Dark Matrix. Click here to download it from the Chrome Web Store. The theme downloads as a browser extension. Download and enable the extension.
Open Developer Tools
No need to restart your browser -- simply open Developer Tools and you'll be good to go!