Fix css for login page has external login

[NguyenThuyLan]

This PR fixes for the issue #19628

[AndyButland]

I'm still seeing a border on the screen when you have external login's configured @NguyenThuyLan:

It's a bit weird, as it's not always there. If I inspect the element it disapears but if I click back on the screen it comes back. So I don't think it's caching. I don't see this once I click to "Sign in with Umbraco".

[iOvergaard]

The border is coming from the native <dialog /> element in the browser, I believe (MDN seems to confirm this). The screen with external login is technically a modal/dialog. You would probably need to interact directly with that element, and that may be hidden inside the <uui-popover-container /> element.