Copy Custom.css to your user style sheet folder
Mac:
~/Library/Application\ Support/Google/Chrome/Default/User\ StyleSheets/Custom.css
Windows Vista/7:
C:\Users\YourUsername\AppData\Local\Google\Chrome\User Data\Default\User StyleSheets\Custom.css
Windows XP: C:\Documents and Settings\YourUsername\Local Settings\Application Data\Chrome\User Data\Default\User StyleSheets\Custom.css
Ubuntu (Chromium):
~/.config/chromium/Default/User\ StyleSheets/Custom.css