--focus-outline-color is set based on whether the theme is brighttext or not. This may lead to unexpected results when a theme applied brighttext to :root, but it isn't a dark theme in all places. One example is this theme https://addons.mozilla.org/en-US/firefox/addon/gradientus/, which uses white text in most of the UI, but the Urlbar is darktext. We apply the dark theme focus outline to a light Urlbar.
We could use currentColor for
--focus-outline-color when a lwtheme is applied. If that provides enough contrast for text, then it would be adequate for borders too. We would still use #00DDFF for our built-in dark theme.
Themes could still set the address bar border color by defining
I'm marking this as proton-address-bar since the address bar is the most visible example of this issue, but it's an issue in other parts of the theme as well.