Change Button / Footer Sign-up

1) I was wondering how I can change the standard Sign Up button to a different button that I created in Illustrator. Is there any way to do this in the css?

Also, I can’t even find where in the css I am able to change the hoover color, shape, etc.. What CSS file should I be updating?

2) How do I add a sign-up box in my footer if I don’t have a “footer” section in my widget. I am using the Horizontal Grid theme from Dessign.net.

Thanks:)

https://www.ads-software.com/plugins/constant-contact-api/