Use correct background color for auth pages

[Boy132]

Closes #4897

While it does not truely fixes the above issue it will at least "hide" it. The real problem behind #4897 is that the React SPA uses the templates/base.core view, even after redirecting to the auth pages. The best fix would probably be to get rid of the templates/base.core & templates/auth.core views and only use the wrapper.blade.php. I guess that would be a change for v2 tho. If wanted I can make a PR to do that or change this PR.