Add Site-Wide CSS
We used to add site-wide CSS by placing a file named local.css in the site root folder. Local.css has been discarded and we have a more elegant method now.
Log in to your site and click on your name in the top right corner.
Select Site-Setup.
Click on Site.
Scroll all the way down and click on Local CSS.
Put in your site-wide code and don't forget to click Save.