So I disagree that this should be a default feature (I think it would interrupt my flow, at least). But it's not so tough to get this in as a default feature.
If you only want it to affect your notebooks going forward, you can make your own stylesheet where you implement this and then set:
to that stylesheet.
Alternatively we can take advantage of the fact that
FrontEnd`FindFileOnPath will scan
$UserBaseDirectory before the system dirs.
That means we can make a whole-sale copy of
"Default.nb" in the
$UserBaseDirectory and just muck-about with that:
Editable -> True,
Saveable -> True
There should then be a way to add in your defintion. Here's a first guess, although it should be possible to do this with things in the
FEPrivate` context. (when I figure that out I'll post it)
Update: FEPrivate confusion
by what I know about the
FEPrivate` context this ought to be working:
Hue[.6, .05, 1],
But for reasons I don't understand it is not. Obviously it doesn't work with
FE`Evaluate, because neither
FrontEnd`EvaluationCell work with it, but I thought in the past it had resolved such things fine, anyway.
Dynamic[If[MemberQ[SelectedCells, EvaluationCell], Blue,
To the cell using Command-Shift-E (on Mac) or Cell > Show Expression
Then restart and you have your change.