Hi,
Thanks for writing in!
You can add a class to your prompt element then target it to change the color of your button.
eg. Add my-prompt in the class field.
You can then add this in custom > Edit Global Css in the customizer
.x-prompt.my-prompt .x-btn {
border-color: #ac1100;
background-color: #ff2a13;
box-shadow: 0 0.25em 0 0 #a71000,0 4px 9px rgba(0,0,0,0.75);
}
Please change the colors in the code
Hope that helps.