Dark mode: text and button fixes

[frosty]

Refs wordpress-mobile/WordPress-iOS#12320.

This PR fixes some dark mode issues with the authenticator:

• The terms and conditions text when you tap sign in is now easier to read and a better size.

• The Next button now shows a white loading indicator in dark mode.

• The username instruction cell used in WPiOS now has a better text color.

To test

• You can use this branch of WPiOS: https://github.com/wordpress-mobile/WordPress-iOS/pull/create?base=wordpress-mobile%3Arelease%2F13.2&head=wordpress-mobile%3Afix%2Fnux-epilogue-dark-mode
• Do a fresh install of the app and tap Sign up. Check you can read the terms and conditions text.
• Sign up a new account and check the epilogue looks okay (these changes are in the WPiOS branch).
• When logged in, navigate to Me > Account Settings > Username and check the instructional text.
• Log out, tap Log in > Continue with WordPress.com > Enter your email address. When you tap next, check the activity indicator color.

[danielebogo]