.woocommerce-form-login .login-btn,  .woocommerce-form-register .register-btn  {
    background: #000 !important;
    transition: 0.3s linear;
}

.woocommerce-form-login .login-btn:hover, 
.woocommerce-form-register .register-btn:hover {
    color: #000 !important;
    background: #fff !important;
}