RE: master 68ae6fa: Improved light/dark colour predicate (bug#41544)

[Drew Adams]

> > But if there are different eyes, screens, whatever,
> > would it make sense to use a defvar, or even a defcustom,
> > instead of a defconst?  Why decide this definitively
> > at design time?  Does it make sense to hard-code it?
> 
> Well, it isn't really hard-coded in the sense that it cannot be changed later
> on, or be turned into a defcustom; both perfectly possible. However, I'd
> rather not get carried away and add yet another customisable variable before
> we have more substantial evidence that it would make sense to do so.

A defvar would be fine.  defconst explicitly signals
the intention that neither users nor code should
change the value.  That's the wrong signal, I think.