Dan,

Not sure about referencing the Styles skin object but you could use the following code to programatically change the style sheet.  You could create a skin object with this code and create a method to pass in the value that you would like the style sheet change to, possibly a value stored in a database table or even a profile property if its on a per user basis.  Hope this helps.

public void ChangeCSS(string CSSValue)
{
 HtmlLink css = new HtmlLink();

        if (CSSValue == "")
        {
            CSSValue = "default";
        }

        string skinPath = PortalSettings.ActiveTab.SkinPath;

        string selectedCSS = skinPath + CSSValue + ".css";

        css.Href = selectedCSS;

        css.Attributes["rel"] = "stylesheet";

        css.Attributes["type"] = "text/css";

        css.Attributes["media"] = "all";

        css.Attributes["id"] = "themestyle";

        Page.Header.Controls.Add(css);

}

protected void Page_Init(object sender, EventArgs e)
{
        ChangeCSS("cssValue");
}

If you have any questions, let me know.

Derek