Theme specific plugin, how?

You can ask for the activation of the theme. You can get the value of the options values templateand stylesheet. The stylesheet get the active style, a benefit if the user use an child theme. As example if ( 'my_theme' === get_option( 'stylesheet' ) ) ....

Also you can use the hooks for template and stylesheet, also s small exmaple.

add_filter( 'option_template', 'fb_theme' );
add_filter( 'option_stylesheet', 'fb_theme' );
function fb_theme( $template="" ) {

    if ( 'my_theme' === $template ) {
        echo 'TRUE';
    }

    return $template;
}