Julian Prein
a8f6973968
In tmux version 3.2 all style options were changed to string
options, making them support formats.[^1]
From that version on, use the pane-borders as indicator for the
{copy,sync}-mode instead of the prefix_highlight plugin.
Keep the pure style fallback, as Ubuntu 20.4 for example still only
ships with version 3.0a.
[^1]: https://github.com/tmux/tmux/commit/f03b61131b34079