Submit Button Background Color

I read this, but can you give me an example of how to change the background button color from grey to green. Where do you put the style and write it?

Edit submit button label: You can easily edit the submit button label into each contact form settings. The class=”cp_subbtn” can be used to modify the button styles. The styles can be applied into any of the CSS files of your theme or into the CSS file “cp-contact-form-with-paypal\css\stylepublic.css”. For further modifications the submit button is located at the end of the file “cp_contactformpp_public_int.inc.php”.

I’ve put this in my child theme css file and the suggested file above.
.cp_subbtn {
color: green;
}
and
(I read somewhere to use this)
.pbsubmit {
color: green;
}
PLEASE GIVE ME A SIMPLE EXAMPLE AND WHERE TO EXACTLY PLACE IT???

I will buy the pro version after I test this your product.
Thanks, Wallace

https://www.ads-software.com/plugins/cp-contact-form-with-paypal/