[Plugin: Constant Contact WordPress Widget] how to style this widget in a child theme?

i have styled my widget contact form (using the no style option) – however i had to add the css in the custom.css file within the wordpress cms. i couldn’t style the widget from my child theme’s css. this is a prob because i need to adjust the style for various screen sizes. YOUR ASSISTANCE IS GREATLY APPRECIATED HERE!

https://www.ads-software.com/extend/plugins/constant-contact-signup-form-widget/