How do I make the button on a configurable list widget the same colour as the other buttons on the form?

My form has a configurable list with a grey button on it.  How can I change that to a green button, the same colour as the other buttons on the form?

Click on the widgets wizard button (small wand like icon in the right corner) and add code as mentioned above:

with button.add for Add button and button.remove for remove button. 

You will need to add it to Custom CSS field:

For a green background, add the following:

button.add, button.remove {
color: #ffffff;
background: green;
}

The color attribute will change the text/+ symbol color, background will change the background color.