Is there a hook for when you switch themes?

There is a ‘switch_theme’ action that runs right after the theme is switched.

function my_on_switch_theme($new_theme) {
    $current_themes = wp_get_themes(); /* Fixed deprecated function */
    $new_theme_info = $current_themes[$new_theme];
    /*
    $new_theme_info should now be an associative array with the following:
    $new_theme_info['Title'];
    $new_theme_info['Version'];
    $new_theme_info['Parent Theme'];
    $new_theme_info['Template Dir'];
    $new_theme_info['Stylesheet Dir'];
    $new_theme_info['Template'];
    $new_theme_info['Stylesheet'];
    $new_theme_info['Screenshot'];
    $new_theme_info['Description'];
    $new_theme_info['Author'];
    $new_theme_info['Tags'];
    $new_theme_info['Theme Root'];
    $new_theme_info['Theme Root URI'];
    ...so do what you need from this.
    */
}
add_action('switch_theme', 'my_on_switch_theme');