Do you ever use your browser’s built in developer tools to inspect the CSS? This makes it easy to find that the default CSS file is wp-content/plugins/speakout/css/theme-default.css
The name field ID is dk-speakout-first-name-1
and the other fields have names based on what they do – the trailing number is the petition ID.
But…I urge you to not edit the included CSS files as they change from time to time and your custom work will be lost.
One solution is to create a custom CSS file (maybe from a duplicate of theme-default.css), read https://speakout.123host.net.au/faq/ for how to do that. But even then, if I improve CSS in the default CSS, you won’t get those updates.
The best solution is to use your themes Additional CSS section and over-ride the default styles with your own settings.