css handler combines the css files in the order that they are listed in style.config. Based on the position of the rule within the output css and looking at the list of files in style.config you should be able to guess/narrow down which file has it.
After editing any css, you need to go to Administration > Advanced Tools > Design Tools > Cache Tool and click the button to reset the skin guid. This will ensure that the browser loads the updated css.
Hope that helps,
Joe