Surprise web developers! Chrome Dev Tools can be customized with a custom user stylesheet :)
This version has been updated for Chrome Version 27.0.1453.93
UPDATE (10/7/13): I think this has broken in Chrome Version 30.0.1599.69 and I'm not sure why...
Place Custom.css in the folder:
~/Library/Application Support/Google/Chrome/Default/User StyleSheets/
C:\Users\<username>\AppData\Local\Google\Chrome\User Data\Default\User StyleSheets\
~/.config/google-chrome/Default/User StyleSheets/