Login Widget: display issues with Twenty Nineteen/Twenty Themes

[jeherve]

### Steps to reproduce

1. On a brand new site using the Twenty Nineteen or Twenty Twenty Theme, install bbPress
2. Add the login widget to your sidebar
3. While logged in, visit your site and view the sidebar.

You'll notice that the button text color is problematic (see attached gif).

I think it would be nice if the widget looked good out of the box with the current default WordPress theme.

[netweb]

Thanks @jeherve, I think some tweaks for both Twenty Nineteen or Twenty Twenty themes would be great