TL;DR:
RGBColor @ {1., 0.972488, 0.797681}
Unfortunately there is no general way to track where options are inherited from and from which style etc., but often taking a look at Core.nb
or Default.nb
with Ctrl+F helps.
core = FileNameJoin[{
$InstallationDirectory, "SystemFiles", "FrontEnd", "StyleSheets",
"Core.nb"
}]
Cases[
NotebookImport[core, _ -> "Cell"],
Cell[StyleData["Placeholder", ___], ___]
]
{Cell[StyleData["Placeholder",
StyleDefinitions -> StyleData["Label"]], ShowSyntaxStyles -> False,
Placeholder -> True, ShowStringCharacters -> False,
StyleMenuListing -> None, FontSize -> 9, FontWeight -> "Plain",
FontSlant -> "Italic",
FontColor -> RGBColor[
0.49321736476691846`, 0.49321736476691846`, 0.49321736476691846`],
Background -> RGBColor[1., 0.9724879835202563, 0.7976806286717022],
FrameBoxOptions -> {BaselinePosition -> Baseline, FrameMargins -> 2,
FrameStyle -> {Directive[GrayLevel[0.79185],
Dashing[{0, Small}]]}},
TagBoxOptions -> {Editable -> False, Selectable -> False,
StripWrapperBoxes -> False}],
Cell[StyleData["Placeholder", "Presentation"], FontSize -> 18],
Cell[StyleData["Placeholder", "Condensed"], FontSize -> 8],
Cell[StyleData["Placeholder", "Printout"], FontSize -> 8]}