Xpath to css selector

Is there a simple way to convert an xpath (copied from Firebug) into a valid css selector which I could use in my style.css to override a certain element?

/html/body/div[4]/div/div[10]/div[3]/div[2]/div/div/div/div/form/div/div/div/fieldset[2]/div/div/div/div[3] is the xpath, but I'm not sure how, or how much of this I need to include? Is div[10] a valid term in a css selector?

Thanks