WordPress plugin enqueue scripts working for js, but not css files

by

The function wp_enqueue_style last argument is the $media and you set it to true so its mean you are doing

<link rel="stylesheet" ... media="1" />

$media (string) (Optional) The media for which this stylesheet has been defined. Accepts media types like ‘all’, ‘print’ and ‘screen’, or media queries like ‘(orientation: portrait)’ and ‘(max-width: 640px)’.

Default value: ‘all’

So you should or remove the last argument so it will be the default all or just set some other media type.

wp_enqueue_style('arena', plugin_dir_url(__FILE__) . 'css/arena.css',
                array(), filemtime( WP_PLUGIN_DIR . 
               '/PluginName/css/arena.css') );