You can easily add new styles in page head with this code inside your controller:
If you still have problem, you can check source code of built-in stylesheet widget in sitefinity https://github.com/Sitefinity/feather-widgets/blob/master/Telerik.Sitefinity.Frontend.InlineClientAssets/Mvc/Controllers/StyleSheetController.cs
The placeholders defined with @HTML.SfPlaceHolder are intended for dragging widgets from the UI.
If you want to register assets at specific places you need sections and the @Html.Stylesheet helper method. Please refer to the corresponding doc article for more details.