How can I change the color of the Tax Toggle button?

You can target any of the parts of the Tax Toggle using simple CSS.

For instance to change the background color:

/* default state */
#wcvat-toggle {
background-color: green;
}

/* activate state */
#wcvat-toggle.on {
background-color: darkgreen;
}

/* hover state */
#wcvat-toggle.on:hover {
background-color: lightgreen
}