When I try to add a new custom tab on the product page, there is an extra line on the top of the tab section. And this extra line displays on each tab except the first tab. I wonder how to fix this problem. Thanks!
Welcome to the community.
To fix your issue, in your browser, right-click on the extra line, Inspect element, find what CSS code is applied, and change either the border or maybe the hr. Or use display: none.
But you are probably not a developer, but it is hard to assist without a link to your store, so please share. And if the frontend is password-protected, please share that too. We could only guess, even if you said Ella theme, that is not enough, what version, what demo? But even that may depend on your content. Do you have content for those tabs?
Welcome to the Shopify Community! Please share your store URL and password (if it’s password-protected), so I can check and provide you with the exact solution.
Best regards,
Devcoder 
OK
Try this, in Edit theme > Product page template go to your section product tabs and add following code in Custom CSS
.tabs-content:not(.tabs-content--row) .tab-content+.tab-content {
border-top: none !important;
}
Code with border is in assets/product-tabs.css but better to update with custom CSS.
In your Shopify Admin go to online store > themes > actions > edit code
Find Asset > base.css and paste this at the bottow of the file:
.tabs-content:not(.tabs-content--row) .tab-content+.tab-content {
border: none;
}
Thank you very much. It works