bug#46493: [feature/pgtk] Low contrast region face

[Po Lu via "Bug reports for GNU Emacs, the Swiss army knife of text editors" <bug-gnu-emacs@gnu.org>]

Lars Ingebrigtsen <larsi@gnus.org> writes:

> It's nonsensical of Emacs to use the Gtk theme for just one colour here.
> It should either use the Gtk theme for the background/foreground
> colours, too, or not use it at all.

It does for the foreground color too, if the contrast is too low.
Otherwise, why would we have a `distant-foreground' specification for
the region face on GTK?

> Emacs composes the colours we use here, and we choose colours that have
> the mixture we want.

We choose the colors specified by the user in his system-wide
preferences by default.

> Our current approach leads to two bugs: With a dark theme, we get a
> nonsensically dramatic region face, and with a light theme, we get a
> region face that has too low contrast.  That's an Emacs bug, and not
> something the user should have to deal with.

I disagree that the contrast is either too low or "dramatic".  And
anyway, I don't think we should be changing long-standing defaults for
personal preferences in this area.