How to Deactivate a Plugin when a Specific Theme is Used

I’m trying to use deactivate_plugins to turn off a Mobile Jquery plugin when the theme is Thematic. It’s not working – what am I doing wrong, please?

<?php
/*
Plugin Name: jQuery Mobile
Plugin URI: https://www.ramoonus.nl/wordpress/jquery-mobile/
Description: jQuery Mobile: Touch-Optimized Web Framework for Smartphones & Tablets. Requires jQuery 1.6 and WordPress 3.2
Version: 1.1.0
Author: Ramoonus
Author URI: https://www.ramoonus.nl/
*/
function deactivate_JQuery_Mobile() {
	$theme_name = get_current_theme();
    $pos = strrpos($theme_name, "Thematic");
	$plugin = plugin_basename( __FILE__ );
	$plugin_data = get_plugin_data( __FILE__, false );

	if ($pos === false){
		if( is_plugin_active($plugin) ) {
			deactivate_plugins( $plugin );
			wp_die( "'".$plugin_data['Name']."' requires Thematic Theme! Deactivating Plugin.<br /><br />Back to <a href='".admin_url()."'>WordPress admin</a>." );
		}
	}
}
function rw_jquery_mobile() {
		// javascript
		wp_deregister_script('jquery-mobile'); // deregister
		wp_enqueue_script('jquery-mobile', plugins_url('/js/jquery.mobile-1.1.0.min.js', __FILE__), array("jquery"), '1.1.0'); // forces jQuery to load

		// css
		wp_deregister_style('jquery-mobile'); // deregister
		wp_enqueue_style('jquery-mobile', plugins_url('/css/jquery.mobile-1.1.0.min.css', __FILE__), false, '1.1.0');
		// css 2
		wp_deregister_style('jquery-mobile-structure'); // deregister
		wp_enqueue_style('jquery-mobile-structure', plugins_url('/css/jquery.mobile.structure-1.1.0.min.css', __FILE__), false, '1.1.0');
		// css 3
		wp_deregister_style('jquery-mobile-theme'); // deregister
		wp_enqueue_style('jquery-mobile-structure', plugins_url('/css/jquery.mobile.theme-1.1.0.min.css', __FILE__), false, '1.1.0');
}
add_action('init', 'rw_jquery_mobile');
?>