I think that this is what you need for that button:
This is the main LINK button of the same type in Moodle
button,
input.form-submit,
input[type="button"],
input[type="submit"],
input[type="reset"],
button,
input.form-submit,
input[type="button"],
input[type="submit"],
input[type="reset"],
button, input.form-submit,
input[type="button"],
input[type="submit"],
input[type="reset"],
button.active,
input.form-submit.active,
input.active[type="button"],
input.active[type="submit"],
input.active[type="reset"],
button.disabled,
input.form-submit.disabled,
input.disabled[type="button"],
input.disabled[type="submit"],
input.disabled[type="reset"],
button[disabled],
input.form-submit[disabled],
input[type="button"][disabled],
input[type="submit"][disabled],
input[type="reset"][disabled] {
background-color: rgba(20, 72, 71, 0.5);
}
And this is the same but for the 'hover', 'focus' and 'active' psudo class
button:hover,
input.form-submit:hover,
input[type="button"]:hover,
input[type="submit"]:hover,
input[type="reset"]:hover,
button:focus,
input.form-submit:focus,
input[type="button"]:focus,
input[type="submit"]:focus,
input[type="reset"]:focus,
button:active, input.form-submit:active,
input[type="button"]:active,
input[type="submit"]:active,
input[type="reset"]:active,
button.active,
input.form-submit.active,
input.active[type="button"],
input.active[type="submit"],
input.active[type="reset"],
button.disabled,
input.form-submit.disabled,
input.disabled[type="button"],
input.disabled[type="submit"],
input.disabled[type="reset"],
button[disabled],
input.form-submit[disabled],
input[type="button"][disabled],
input[type="submit"][disabled],
input[type="reset"][disabled] {
background-color: rgba(20, 72, 71, 0.5);
}