How do I add CSS styles to an ASP.NET UserControl from the code-behind?

this.Style.Add ( /*...*/ )

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What do I need to do to add a style to it?

Think about the nature of UserControl - it's intended to contain other ASP.NET controls and that's why you cannot set CSS for whole control.. instead you just need to specify styles for each child control individually. You can also take a look at the ApplyStyleSheetSkin method.

You may wrap your user control into another html container (div for example) and set style on container.

If your user control is inheriting from webcontrol simply use the system.web.ui.webcontrols.webcontrol.cssclass (msdn). Example in the link.