Custom CSS pages are pages on your wiki that allow you to add CSS which will be applied to your entire wiki. Custom CSS must follow the customization policy. You can set your own personal styles using personal CSS which will only apply to you.
Site Wide Pages[]
You can go to Special:CSS, though it currently leads to MediaWiki:Common.css which is the default CSS. Alternatively, you can go to MediaWiki:Fandomdesktop.css (for the FandomDesktop skin only).
Additionally, you can create any page in the MediaWiki namespace ending in .css and import it into one of the above pages.