I need a bit more help to understand. I copied the code to userContent.css. I have put logo.gif in the chrome folder. I changed the color to background-color:#FAEBD7; and I changed the line with logo.gif to url("chrome://logo.gif"); because there were no folders in chrome. Nothing changes when I do a new tab and get an empty window with about:blank.
Sorry, but this old guy need more detailed instructions, please.
You should also copied the new logo in chrome\embed.jar and keep url("chrome://global/content/logo.gif"
See a good explanation in: this thread
Thanks for your explanation, good job