Hi there,
You can try adding this CSS to your site:
a, button, input {
transition: background-color .1s ease-in-out;
}
.gb-button-wrapper .gb-button {
transition: .2s background-color ease-in-out, .2s border-color ease-in-out,.2s opacity ease-in-out,.2s box-shadow ease-in-out !important;
}
Make sure to clear any caches afterward.
Or you can simply ignore those diagnostics, they are not causing any issues, and i am not sure why Google flags them as Unsupported CSS Property: color
when color is a supported according to the CSS spec.