button text color

Hi,

You can use this css code and add it into Customize of theme:

.group-button .add_to_cart_button,
.group-button .added_to_cart,
.group-button .product_type_simple,
.group-button .product_type_external,
.group-button .product_type_grouped{
color: #fff;
}

You also need to backup this code if you want to update the Main theme in the future.
Regards,
Yame