Preview CSS on your live site with Charles proxy
First enable local mapping. Under "Tools" > "Map Local".
Charles enable map local
Then select the CSS file you want to override with your local copy.
Too easy. When you next refresh your browser you will see the changes you made to your CSS. You now have a great way to double-check your changes before go live.
Have a good explore around all the features of Charles, it's a great tool to have in your toolbox.