Both CSS and JS can be either personal or site-wide. Personal code is that which affects your account only. Site-wide code applies to an entire wiki, such that all visitors will see its effect. Under Fandom’s customization policy, not all code is allowed to be site-wide, whereas you can do whatever you want with personal code.

Did you know that the base MediaWiki software actually has documentation? Don't be ashamed if you don't! Instead, read the introduction on MediaWiki.org!