CssFile
data class CssFile(val filename: String, val id: String = "", val origin: AssetOrigin = AssetOrigin.REVEAL_ASSETS)
A CSS file referenced from the generated <head>. A relative filename resolves against origin; an absolute, external, or data: value is emitted as written. The optional id is emitted as the <link>'s id attribute — used internally to tag theme / highlight stylesheets.