Allow styling of borders separately from default files

[nout]

One big issue that's preventing creation of better looking skins is that borders are always styled the same color as files (and other default foreground text).

One partial trick is to introduce a new filehighlight that captures all files and style that separately in the skin.
This is what I did in ​https://github.com/notnout/mashdark , but that's hacky and users have to download my filehighlight.ini definition. Note that this trick doesn't work in dialogs and menus.

So the request here is to add a new param in the style definition that would allow styling of the borders (ideally including the "buttons" for hidden files).