Hi Heather-
You’ll want to add the CSS to the Theme Customizer.
The button looks a little different than you described, I want to double-check that we’re talking about the Get In Touch button near the top of your contact page?
Here’s the code that worked for me on that button (the hover text is already black):
a.btn.btn-color-2.btn-large.btn-block:hover {
text-decoration: underline !important;
background-color: lightgray;
}
This will use the system default “Light Gray” color, but you can replace that with a different color code if you need to.