There are so many articles about `cookies` and `localStorage` (setting, deleting...), but it's unclear for me where I should place some code. 

Namely, I'm trying to change css stylesheet clicking on a button, but cannot save that choice for the next time the page is loaded. Something like: 

params.php

```
if (user clicked on "btntheme") {
    $theme = 'green.css';    
}else{  
    $theme = "blue.css";
};

```

index.php 

```
<?php include ("params.php");?>
<head>
<link id="link01" rel="stylesheet" href="<?php echo $theme;?>">
</head>

<body>
<div id="btntheme">THEME</div>
</body>

```

JS 

```
$('#btntheme').click(function(){
    $('#link01').attr('href', 'green.css');
});

```

This changes style but only for the current session. How can I tell to `params.php` that in the future `$theme` is `green.css`, and not `blue.css`?