since there should be only 1 form tag on the page, you could just use the CSS selector "form" instead of ".text" to select it.
Otherwise you can add the class using javascript:
jQuery('form').addClass("text");
Or you could add a script runat="server" block in your master page like this:
<script runat="server">
protected void Page_Load(object sender, EventArgs e)
{
Page.Form.Attributes["class"] = "text";
}
</script>