Changing Submit Button for Jetpack Email Subscription Widget

I’ve Google and found a bunch of tutorials for how to change the color/shape of the submit button for the Jetpack email subscription widget but I can’t seem to get it to work.

Does anyone know what CSS code I have to enter? I just want something basic that looks a little more polished than the default, something like the button in the below:

http://www.theblogmarket.co/wp-content/uploads/2016/02/jetpack-subscribe_00.png

Thank you!