CSS color scheme switching

[eluser]xwero[/eluser]
why can't you do something like

Code:

// controller
if(isset($_POST['color']))
{
setcookie("cookie[theme]", isset($_POST['color']);
}
if($this->input->cookie('theme'))
{
  $this->load->view('page',array('csstheme'=>$this->input->cookie('theme'));
}
else
{
setcookie("cookie[theme]", 'green');
}
// view
<link rel="stylesheet" type="text/css" href="theme_<?php echo $this->input->cookie('theme'); ?>.css" />

I don't understand why you would want to have an dynamic css file to make a theme switcher?