Consolidate focus ring styles/colors

[craigcook]

Some elements, namely buttons and form fields, get a colored halo when they receive focus. There are a few different styles defined in https://github.com/mozilla/protocol/blob/main/assets/sass/protocol/includes/forms/_index.scss#L31-L35

We should unify focus styles for consistency across elements, and probably prefer opaque token colors rather than mixing or transparent. We may still want a different color for light and dark themes.