We've improved the customer account login experience by automatically applying your branded domain to the login page. Previously, even if you had configured a custom domain for customer accounts, the login page would use a default Shopify domain - https://shopify.com/authentication/<id>/login. With this update, your customer account login page automatically inherits your configured custom domain - https://account.yourdomain.com/authentication/login
Customers will see a consistent branded experience from login through to their account pages. This change applies to all merchants who have configured a custom domain for their customer accounts. For merchants who haven't set up a custom domain yet, you can configure one through your store's domain settings to take advantage of this improved branding experience.
Learn more about configuring a custom domain for your customer account pages in the Shopify Help Center