I’m having the same issue, I’m using firebug in firefox and using windows 10. Theme is Sydney, I can easily make header background transparent in Firebug but the location of the file is unknown, somewhere in root with line#4.
I’ve found this through search but it didn’t helped me as this gives me multiple results instead of one. I hope this will help you.
http://stackoverflow.com/questions/6128369/how-do-i-get-css-source-file-path-from-firebug
Here someone suggest we should create new custom.css for overriding theme file.
http://vanillaforums.org/discussion/24391/trying-to-edit-css-files-with-firebug-why-doesnt-it-tell-me-which-css-files-i-need-to-manipulate
