public void SetFonts()
{
    string strHtml = "<span style="font-size: 10px; ">Hi This is just a section of html text.</span><span style="font-family: 'Verdana'; font-size: 10px; ">Please help</span><span>me add style attributes to span tags.Thank You</span>";
    HtmlDocument document = new HtmlDocument();
    document.LoadHtml(strHtml);
    SetFonts(document.DocumentNode);
    document.Save(Console.Out);
}


public void SetFonts(HtmlNode node)
{
    foreach (var item in node.Elements("span"))
    {
        var style = item.GetAttributeValue("style", null);
        if (style != null)
        {
            if (!style.Contains("font-family"))
            {
                var newValue = style + "font-family: 'Verdana';";
                item.SetAttributeValue("style", newValue);
            }
        }
        SetFonts(item);
    }
}