So, it's not possible to do this with an extension?
Guess I have to alter the BlogEngine.NET code then to make this possible?
Okay, I found a way:
Added a Pipeline class to add Action<T> to a List.
This list will be executed on Page_PreRender of a MasterPage.
ContentPlaceHolder c = page.FindControl("HeadContent") as ContentPlaceHolder;
LiteralControl ltr = new LiteralControl();
I am able to add the CSS just once to my Page.
Extension CSSSwitch is born ... somehow