Go to: Confluence Admin > Look and Feel > Stylesheet > Edit
This will affect all pages in the entire wiki. This will also remain as part of Confluence to run even if Refined is disabled or uninstalled.
Global level with Refined
Go to: Refined Administration > Themes > scroll to the bottom of the page to find Custom CSS and HTML
This will affect all pages in the entire wiki, but it is packaged with Refined, meaning that if you uninstall or disable Refined, this will not be applied.
Go to: Browse > Space Admin > Look and Feel > Stylesheet > Edit
This will affect all pages in the active space.
A RefinedThemed Refined for Confluence acts a Confluence in general when it comes to permission, you can read more about who can apply css here: Styling Confluence with CSS.