Adding CSS class through ASPX code

If you want to use a control on serverside make it runat=server or even better: use a servercontrol in the first place. In this case you should use a Panel which is rendered as a div:

<asp:Panel ID="PanelID" CssClass="classMe" runat="server"></asp:Panel>

or from codebehind:

protected void Page_Load(object sender, EventArgs e)
{
    PanelID.CssClass = "classMe";
}

But if you want to stay with your div:

<div id="DivID" runat="server"></div>

codebehind:

protected void Page_Load(object sender, EventArgs e)
{
    DivID.Attributes.Add("class", "classMe");
}

Change the div to something like:

<div id="classMe" runat="server"></div>

Then in your code-behind ASPX.CS file, you can access it.

E.g.

protected void Page_Load(object sender, EventArgs args)
{
    // Access it like this to set a CSS class.
    classMe.Attributes["class"] = "MyCssClass";

    // Access it like this to directly add an inline CSS style:
    classMe.Style.Add("color", "red");
}