I've had several questions in comments about how to add CSS files using the Simplest-Safest-Most-Future-Proof-Way to customize your SharePoint and SharePoint Online using UserCustomActionConfigPage
This is something I wanted to talk about from day 1 of this series - as I did cover you can add more than 1 CSS files, and specify their ranking via the Sequence number.
Because UserCustomAction doesn't create a link for CSS references, the way to do this is via the ScriptBlock and create a LINK element and attach it on page creation.
How to use this?
Grab the latest configure-page.aspx from Github.
Reference a CSS file instead of a JS file. The ID is optional, but will be respected if specified.
The Sequence is honoured - 1001 will follow 1000.
Result - the CSS reference is loaded into the page.