I've been asked to restyle the css of a WordPress website frontend. The website can't be in maintenance mode while working on it as it must stay up and running all the time. I assume that if I can add a small and unobtrusive condition in the header allowing the administrator to see a different css file than the users, I could edit the css and test the result while the users still see the original version of the website. This way I could work on the design without anyone noticing.
Can anyone tell me which WordPress function I should call on the header to switch the css file if and only if the user is an administrator?
The best would be not to edit the "functions.php" file or activate any plugin as I really don't want to mess with the website as it is (few chances but chances anyway). I would rather edit the "header.php" file in the theme folder in order to be sure nothing bad happens.