Can we add css file for composite custom user control and apply it to child controls ?

I have create a custom calendar control with the help of web child controls(Text Box, Image Button and Calendar). And I have added the CSS file in this project as per your solution.
Now I want to apply some class belongs to this CSS file to my child controls(Text Box and ImageButton) in server control.
So How Can I apply ?
Code is Here
public class FirstCustomControl : CompositeControl
{
TextBox txtbox;
ImageButton imgbtn;
Calendar calender;

protected override void CreateChildControls()
{
Controls.Clear();
txtbox = new TextBox();
txtbox.ID = "txtCalender";
txtbox.Width = Unit.Pixel(150);

imgbtn = new ImageButton();
imgbtn.ID = "Imgbtn";

calender = new Calendar();
calender.ID = "Calender";

this.Controls.Add(txtbox);
this.Controls.Add(imgbtn);
this.Controls.Add(calender);
}
protected override void OnPreRender(EventArgs e)
{
Page.ClientScript.RegisterClientScriptInclude("animation",Page.ClientScript.GetWebResourceUrl(this.GetType(),"FirstCustomControl.css.animate.css"));
Page.ClientScript.RegisterClientScriptInclude("MLM", Page.ClientScript.GetWebResourceUrl(this.GetType(), "FirstCustomControl.css.VishalMLMStyle.css"));

}
protected override void AddAttributesToRender(HtmlTextWriter writer)
{
writer.AddStyleAttribute("Color", "red");

}
protected override void Render(HtmlTextWriter writer)
{
txtbox.RenderControl(writer);
imgbtn.RenderControl(writer);
calender.RenderControl(writer);
}
}

Calender is object of Calendar class...