The layout of the plugin can all be configured using CSS. The good news is you are already using a class that can make thins prettier in Subscribe2 further up the page for your login widget.
To reuse this for the subscribe2 Widget, visit the Widget page in the WordPress admin area and open up the Subscribe2 widget to see the settings. In there you will see a space to enter a class name and it's currently set as 'search'. Change this to 'login' and have a look at the front of your site again.
If you want to make further changes then it might be best to define a new class name in your themes stylesheet and define your CSS in there for the Subscribe2 widget.