The Jetpack subscription widget doesn't have a CSS file associated with it (as far as I can tell). The CSS elements are embedded into the PHP files.
Your best bet is to make the actual CSS changes to the <div> elements within the subscriptions.php file in the modules folder.
For instance, in the "// Check for subscription confirmation" I changed this:
<div style="background-color: #ee8b16; border: 1px solid #E6DB55; padding-left: 5px; padding-right: 5px; margin-bottom: 10px;">
<div style="background-color: #ffffff; color: #000000; border: 1px dotted #26201b; padding-left: 5px; padding-right: 5px; margin-bottom: 10px;">
The only difference being that during a plugin update, you may need to re-add it.
you can add your own CSS and only have to put the class names back in during an update