Changing the CSS with a plugin

You’re using the wrong hook to enqueue your styles:

add_action('admin_enqueue_scripts', 'seizoenplugin_enqueue_styles' );

As the name suggests, admin_enqueue_scripts is for enqueuing scripts and styles in the admin. To enqueue scripts on the front end use wp_enqueue_scripts:

add_action('wp_enqueue_scripts', 'seizoenplugin_enqueue_styles' );