Front End options

Option hierarchy

Quoting John Fultz:

Options work on an inheritance model. ... It all starts someplace. That would be the hard-coded option values in the front end C code. The root of the inheritance of options. Then one typically thinks of $FrontEnd as being the next level down, but there's another level in between. That is $DefaultFrontEnd. So, at the global level, it's -> $DefaultFrontEnd -> $FrontEnd -> $FrontEndSession. But only values set to $FrontEnd get written to your preferences file.

In addition to these global settings many Front End options (or their equivalents) can be set at the Notebook level, with e.g. EvaluationNotebook[], and these, when set, take local priority.

Settings made to the Notebook are saved in the Notebook (.nb). Settings made to $FrontEnd are saved in:

FileNameJoin[{$UserBaseDirectory, "FrontEnd", "init.m"}]

Settings to either $FrontEndSession or $DefaultFrontEnd do not persist between sessions. Settings made to $FrontEndSession override those to $FrontEnd for the duration of the session, while $DefaultFrontEnd are only used if the same option is specified nowhere else.

Setting and recalling option values

$FrontEnd and Notebook options can be set and cleared using the Option Inspector with the menu selections Global Preferences and Selected Notebook respectively. To clear an option click the x next to its option name:

Options can be set and recalled at all levels with SetOptions and Options.
For example one could make the following settings:

SetOptions[$DefaultFrontEnd,     FontColor -> Red    ]

SetOptions[$FrontEnd,            FontColor -> Orange ]

SetOptions[$FrontEndSession,     FontColor -> Magenta]

SetOptions[EvaluationNotebook[], FontColor -> Blue   ]

Many options are aslo accessible and configurable through CurrentValue. An unqualified CurrentValue will show the value lowest in the hierarchy (with the highest priority):

CurrentValue[FontColor] // InputForm

RGBColor[0, 0, 1]  (* blue *)

A qualified CurrentValue will show the other settings as well:

CurrentValue[#, FontColor] & /@
  {$DefaultFrontEnd, $FrontEnd, $FrontEndSession} // InputForm

{RGBColor[1, 0, 0],    (* red     *)
 RGBColor[1, 0.5, 0],  (* orange  *)
 RGBColor[1, 0, 1]}    (* magenta *)

The qualified form may be used to set option values:

CurrentValue[EvaluationNotebook[], FontColor] = Green;

If the Notebook option is cleared through the Option Inspector, or a new Notebook is opened, the next setting up the hierarchy is used:

CurrentValue[FontColor] // InputForm

RGBColor[1, 0, 1]

After Mathematica is restarted the $FrontEnd setting persists:

CurrentValue[FontColor] // InputForm

RGBColor[1, 0.5, 0]

---

As David Creech noted in a comment it is possible to use Inherited as an option value to un-set that option and revert to the next higher level of the hierarchy. Example:

SetOptions[EvaluationNotebook[], FontColor -> Blue]

Options[EvaluationNotebook[]]

{FontColor -> RGBColor[0, 0, 1], FrontEndVersion -> . . .}

SetOptions[EvaluationNotebook[], FontColor -> Inherited]

Options[EvaluationNotebook[]]

{FrontEndVersion -> "10.1 for Microsoft Windows . . .}

Note that the Option has been entirely removed from the Notebook options list.

A useful point on recalling style definitions. These may be queried at the stylesheet level using StyleDefinitions:

CurrentValue[$FrontEndSession, {StyleDefinitions, "Input", FontColor}]

Automatic

They cannot, however, be set so easily (but I've got code to do that e.g. here):

CurrentValue[$FrontEndSession, {StyleDefinitions, "Input", 
   FontColor}] = Red

Automatic

One useful thing about this is we can inspect more interesting options programmatically using this:

inspectBoxOptions[element_] :=

 With[{boxo = 
    StringTrim[
      StringReplace[SymbolName[element], "Framed" -> "Frame"], 
      "Box"] <> "BoxOptions"},
  CurrentValue[$FrontEndSession, {StyleDefinitions, "Input", boxo}] /.

      s_Symbol?(StringStartsQ[SymbolName[#], boxo] &) :> 
    StringTrim[SymbolName[s], boxo]
  ]

inspectBoxOptions@Checkbox

{"BaseStyle" -> {}, "DefaultBaseStyle" -> "Checkbox", 
 "Enabled" -> Automatic, 
 "Appearance" -> {Automatic, None, "Normal", Automatic}, 
 "Background" -> Automatic, "ImageSize" -> All, 
 "ImageMargins" -> Automatic, "FrameMargins" -> Automatic, 
 "ContentPadding" -> True, "BaselinePosition" -> Automatic, 
 "Alignment" -> {Automatic, Automatic}, "AutoAction" -> False, 
 "ContinuousAction" -> False, "Momentary" -> False}