Re: Changing the no-toolkit scrollbar thumb color. (minor issue)

[Richard Stallman]

    This does not go far enough.  You should not be able to enter a
    customization buffer when customization does not work.

Regarded as a general principle, that would be very hard to carry out.
Emacs has many customizable options that only have an effect in
certain situations.  In principle, we could try to disable the ability
to customize them when not in those situations, but that would be a
lot of work.

It would also be wrong, in a sense, because customizing these options
and saving the changes can be meaningful even if the current situation
is such that changing the option has no effect.