As an update I was able to see some effects adding to: custom.css
p.readmore a.btn {
color: #2CB42C;
background: #b2b2b2;
}
The background change works, but then color is not applied, (purposefully a crazy color for testing)
Assuming "color" would change the text color, which I'm not sure about....
Code is not my thing so forgive me if the reason is obvious to others 🙁