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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}|>]