change css element value dynamically

If you want to change the CSS element at runtime through server side code then you need to convert that control into server control by adding 'runat=server' OR inject JavaScript from code behind to change the style.

Option #1: You define the header as server control and provide ID to the header
You can directly, access the CssClass property of the control and set to desired one

Option #2: You inject the JavaScript into page from code behind
You can do it using:
MSDN: RegisterClientScriptBlock Method[^] OR
MSDN: RegisterStartupScript Method[^]

In the JS method, find the header element and access it's style-backcolor attribute to modify it.

Try!

There are several ways we do,

Simplest way

textBox1.Attributes["class"] = "class name";
//textBox1 is a Text Box Server Control and "class name" is your custom class

Another way is to use Web Extension,

static class WebControlsExtensions
 {
     public static void AddCssClass (this WebControl control, string cssClass)
     {
         control.CssClass += " " + cssClass;
     }
     public static void RemoveCssClass (this WebControl control, string cssClass)
     {
         control.CssClass = control.CssClass.replace(" " + cssClass, "");
     }
 }

ctl.AddCssClass("ReadOnly");
ctl.RemoveCssClass("ReadOnly");

Hope this helps.
cheers