Put this into your chrome User Stylesheets folder to have instant satisfaction.

*originally nabbed from http://darcyclarke.me/design/skin-your-chrome-inspector/

Mac:

~/Library/Application\ Support/Google/Chrome/Default/User\ StyleSheets/Custom.css

PC:

C:\Users\YourUsername\AppData\Local\Google\Chrome\User Data\Default\User StyleSheets\Custom.css

Ubuntu (Chromium):

~/.config/chromium/Default/User\ StyleSheets/Custom.css