Add files to wordpress admin panel footer

You can use $pagenow global variable for this, which would obviously return widgets.php when in widgets dash.

If you want to enqueue CSS/JS by paths:

add_action( "admin_enqueue_scripts", "wpse_250923_enqueue_scripts" );

function wpse_250923_enqueue_scripts() {
    global $pagenow;
    // check for widgets
    if ( "widgets.php" !== $pagenow )
        return; // nothing to do here

    // enqueue CSS
    wp_enqueue_style( "my-css-handle", "path/to/some/css/file.css" );

    // enqueue JS
    wp_enqueue_script( "my-js-handle", "path/to/some/js/file.js", array('jquery') /*dependencies*/ );
}

Otherwise if all you have is some inline JavaScript and/or inline CSS (not stored in a file) then assign admin_head for the CSS and admin_footer for the JS:

add_action( "admin_init", "wpse_250923_enqueue_scripts" );

function wpse_250923_enqueue_scripts() {
    global $pagenow;
    // check for widgets
    if ( "widgets.php" !== $pagenow )
        return; // nothing to do here

    // enqueue CSS
    add_action( "admin_head", "wpse_250923_print_css" );

    // enqueue JS
    add_action( "admin_footer", "wpse_250923_print_js" );
}

function wpse_250923_print_css() {
    print '<style>/*CSS tweaks*/</style>' . PHP_EOL;
}

function wpse_250923_print_js() {
    print '<script type="text/javascript">/*JS goes here*/</script>' . PHP_EOL;
}

Hope that helps. Happy New Year in advance!