Hi Salman,
Now that we have the login API, you can create your custom login page, and add a link from the signup custom page to the login page (eg., "already a member? click here to login ).
In addition, in the member signup settings panel, under the “which option shows first” section you need to select " New member signup ".
This way the default login page will never be triggered by an external application / restricted page / login bar, and only the custom signup page will be opened with a link to the custom login page.
Hope this helps.
Let me know if you have any questions.
Thanks