[noppid]

Since this will be in the header, you likely need to make a global start hook plugin. Try putting the above code in a global start hook.

hook code global start

PHP Code:

 // BEGIN STYLESWITCHER CODE
    if(!isset($reqPath)){ $reqPath = "./"; }
    require_once($reqPath ."_custom_includes/Styleswitcher.php");

    $ss = new Styleswitcher();
    $ss->addStyle("basic", "basic.css", "", "", true);
    $ss->addStyle("blue", "blue.css");
    $ss->addStyle("green", "green.css");

    ... more identical stuff here ...

    // End Styleswitcher code
    $SS_script = $_SERVER['PHP_SELF'];
    $SS_normal = $ss->printSetInputChecked("fonts", "normal")
    $SS_large = $ss->printSetInputChecked("fonts", "large"); 
    eval('$SS_template = "' . fetch_template('SS_template') . '";'); 


SS_template

<form action="_custom_includes/switcher.php" method="post">
    <!-- Automatically redirect to the referer -->
    <input type="hidden" name="referer" id="referer" value="$SS_script" />
    <input type="hidden" name="inputStyle1" id="inputStyle1" value="fonts" />
    <input type="hidden" name="inputStyle2" id="inputStyle2" value="style" />
  
    <strong>Font Style:</strong><br />
    <input type="radio" name="fonts" id="fontStyle1" value="normal"$SS_normal /> <label for="fontStyle1">Normal text (smaller)</label>
    <input type="radio" name="fonts" id="fontStyle2" value="large"$SS_large /> <label for="fontStyle2">Large text</label><br />

    ... addtl. form elements here ...

    <input type="submit" name="setChanges" value="Change styles" />
</form>

and put $SS_template in the header template. I think that should work. I have not tested it though.

don't forget to cache the template.