Rrerku

The answer by @Kuba changing color of error messages did not seem to work for me (MMA 11.0.1 Win 10 64-bit).

I couldn't work out why and implemented something else that does work for me, but it seems very ugly and I'm sure I could learn much by seeing how others would improve on it.

I think @Kuba's answer should have worked because I created the Default.nb as proposed in $UserBaseDirectory and my stylesheets ultimately inherit from it (Default.nb -> JM Stylsheet -> JM Clearer for TeamViewer).

However, I also directly modified my own stylesheets by using the "enter a style name" box, entering MessageMenuLabel and MessageText and styling them appropriately. The new styles worked and I saved stylesheets but on restarting MMA these styles were gone again.

Finally I wrote this (my 1st attempt to programmatically manipulate style definitions) making further use of @Kuba's answer to this question as follows

(* To preserve the current stylesheet information it has to be
plucked out of the StyleDefinitions; 1st time this is OK as the
StyleDefinitions = just a notebook name, but after adding items it
gets messy and we need to extract the stylesheet notebook name to reapply it.
*)
sdef = CurrentValue[EvaluationNotebook[], StyleDefinitions];
If[! StringQ[sdef], (* 
 this is typically just the filename of a stylesheet notebook, 
 but if it isn't... *)
 sdef = ToString[sdef]; 
 sdef = StringCases[sdef, "StyleDefinitions -> " ~~ __ ~~ ".nb]]", 
 1]; 
 sdef = StringReplace[
 sdef[[1]], "StyleDefinitions -> " -> "", "]]" -> ""] 
 ];
SetOptions[EvaluationNotebook[], StyleDefinitions -> Notebook[
 Cell[StyleData[StyleDefinitions -> sdef]],
 Cell[StyleData["MessageMenuLabel"], Bold, 
 FontColor -> RGBColor[N[174/255], 0.1, 0], 
 FontSize -> 
 CurrentValue[StyleDefinitions, "Output", "FontSize"]],
 Cell[StyleData["MessageText"], 
 FontColor -> RGBColor[0.1, 0.1, 0.1]]
 
 ]
 (* last line needed per Kuba's Programming scripts to create and modify stylesheets answer*)
 /. s_Symbol /; Context[s] === "Global`" :> 
 Symbol["FrontEnd`" <> SymbolName[s]]] 
(* Do something illegal to check the message appearance... *)
1/0

Questions Why might the straightforward approach not have worked, and - for educational purposes - how should it be done programmatically & idiomatically?

I worked on this a while back and found that it was just too messy to really work with the Notebook expression.

Here's a better approach: 1) pull the NotebookObject's stylesheet 2) determine if the cell style you want to edit is in there 3) edit that style or make a new cell to edit

Here's a quick imp for that:

nbStyleSheet[nb_] :=

 With[cv = CurrentValue[nb, StyleDefinitions],
 If[! MatchQ[cv, _Notebook], 
 SetOptions[nb,
 StyleDefinitions ->
 Notebook[Cell[StyleData[StyleDefinitions -> cv]],
 StyleDefinitions -> "PrivateStylesheetFormatting.nb"
 ]]
 ];
 Lookup[NotebookInformation[nb], "StyleDefinitions"][[1]]
 ];

findStyleData[nb_, 
 stylePattern : _?StringPattern`StringPatternQ : "*"] :=

 Module[cells = Cells[nb],
 Association@
 MapThread[
 Replace[
 #,
 
 Cell[
 StyleData[
 name_String?(StringMatchQ[stylePattern]), ___], ___] :>
 (name -> #2),
 _ -> Nothing
 
 ] &,
 
 NotebookRead[cells],
 cells
 
 ]
 ];

editStyleCell // Clear
editStyleCell[nb_, styleCell_, styleEdits_] :=
 MathLink`CallFrontEnd@
 FrontEnd`SetOptions[styleCell, styleEdits],
 (* this is a hack to make these edits apply immediately *)

 FrontEnd`SelectionMove[styleCell, All, Cell],
 FrontEndToken[nb, "ToggleShowExpression"],
 FrontEndToken[nb, "ToggleShowExpression"]
 

makeMissingStyles[nb_, names_] :=
 MathLink`CallFrontEnd@
 FrontEnd`SelectionMove[nb, After, Notebook],
 FrontEnd`NotebookWrite[nb, Map[Cell[StyleData[#]] &, names]]
 

styleSheetEdit[notebook_, styleEdits_?AssociationQ] :=
 Module[
 
 names = Keys[styleEdits],
 nb = nbStyleSheet[notebook],
 cells,
 missing
 ,
 cells = findStyleData[nb, Alternatives @@ names];
 missing = Complement[names, Keys@cells];
 If[Length@missing > 0,
 makeMissingStyles[nb, names];
 cells = Join[cells, findStyleData[nb, Alternatives @@ missing]]
 ];
 MapThread[
 editStyleCell[nb, #, #2] &,
 
 cells,
 styleEdits
 
 ];
 ];
styleSheetEdit[styleEdits_?AssociationQ] := 
 styleSheetEdit[EvaluationNotebook[], styleEdits];

Let me know if you have questions. Meantime you can edit notebook stylesheets like this:

styleSheetEdit[<|"Input" -> FontColor -> Pink|>]

CurrentValue[EvaluationNotebook[], StyleDefinitions]

Notebook[Cell[StyleData[StyleDefinitions -> "Default.nb"]], 
 Cell[StyleData["Input"], FontColor -> RGBColor[1, 0.5, 0.5]], 
 Visible -> False, 
 FrontEndVersion -> "12.0 for Mac OS X x86 (64-bit) (April 8, 2019)", 
 StyleDefinitions -> "PrivateStylesheetFormatting.nb"]

You can revert changes by setting them to Inherited:

styleSheetEdit[<|"Input" -> FontColor -> Inherited|>]