I use some themes for Google Developer Tools from this website: http://devthemez.com/themes/chrome-developer-tools
However, after the 32.0.1700.76 m update, all them
⌘ + option/alt + i
⌘ + shift + p
(make sure to click on the devtools before doing this keyboard shortcut)dark
and you will see an option to switch to dark theme
You can also follow the directions here
Developer mauricecruz has made a nice tool for making custom Chrome DevTools themes.
https://github.com/mauricecruz/zero-base-themes
It's a lot easier than writing a CSS file (in my opinion).