When designing user specific layout, besides themes and .css files we have also option of switching master pages. To do so we should write below code in PreInit event . ( this change can be occur just in PreInit, since after that, merging phase of content and master pages is already happened )

protected void Page_PreInit(object sender, EventArgs e)
{
     this.MasterPageFile = "~/otherMasterPage.master";
}