To change the woocommerce tab border color, just add the following CSS in the “Appearance => Customize => Additional CSS” box:

.woocommerce div.product .woocommerce-tabs ul.tabs:before {
   border-bottom-color: #dddddd;
}

.woocommerce div.product .woocommerce-tabs ul.tabs li {
   border-color: #dddddd;
}

.woocommerce div.product .woocommerce-tabs ul.tabs li:after,
.woocommerce div.product .woocommerce-tabs ul.tabs li:before {
   border-color: transparent;
}

And as for password protected design and button add the following CSS:

input[type="submit"] {
   background: transparent;
   background-color: #008080;
   color: #ffffff;
   border: none !important;
   box-shadow: none !important;
   transition: all 0.3s ease-in-out;
   padding: 9px 20px !important;
}

input[type="submit"]:hover,
input[type="submit"]:focus {
   background: transparent;
   background-color: #000000;
   color: #ffffff;
}

.post-password-form label input {
   box-sizing: border-box;
}

@media screen and (max-width: 560px) {
   .post-password-form label input {
      width: 100%;
      margin-top: 7px;
   }

   input[type="submit"] {
      margin-top: 10px;
   }
}

You can change the colors as you want. Let me know if this worked for you. Thank you.

Regards,
Sujeet