Changing colour of Send Button?

Meg,

firstly thanks for the great plugin, works much better than other contact forms I tried.
The only thing I would like to be able to do is to change the colour of the send button (it currently shows as yellow on my illustrator friends theme) and for it to match it needs to be green.

website is http://www.bykatemcguire.com

Any help would be much appreciated.
Thanks in advance

Nic

https://wordpress.org/plugins/clean-and-simple-contact-form-by-meg-nicholas/