notebooks - Why MMA cannot apply the correct style of unit?

When I select

Style-> Style-> DisplayFormulaNumbered

Then I type some formula show as below:

I found that the font of formula is not Times New Roman,so I select the unit ,then select Style->Font, which shows that the font of formula is courier new.

Question

How to revise this condtion? or is it the default style of DisplayFormulaNumbered

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Update

Thanks for Sjoerd C. de Vries's solution.

When I select

• Format > StyleSeet > Book > Textbook

then I select

• Format > Style > EquationNumbered

Lastly, I input some formula shown as below (it show the correct formula style)

I looked up the definition of EquationNumbered,then I found a difference DefaultFormatType -> TraditionalForm

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So I execute some operations as below

• 
  

  Format > StyleSeet > Default

  
  


• 
  

  Format > Edit StyleSeet

  
  


• 
  

  copying the DisplayFormulaNumbered style definitions from the Default.nb parent

  
  



• 
  

  

  
  

However, the result of formula remains keep the font of courier new.

Answer

The problem is that the way it has been configured DisplayFormula creates cells with the structure Cell[BoxData[...],...] whereas what you preferably want (IMO) for displaying formaulas is Cell[TextData[...],...].

So the first step would be to make this change to your private stylesheet:

DefaultFormatType->DefaultTextFormatType,

or you could also make the cell style inherit from the Text style:

Cell[StyleData["DisplayFormula",StyleDefinitions -> StyleData["Text"]],
 CellMargins->{{66, Inherited}, {Inherited, Inherited}},
 StripStyleOnPaste->True,
 DefaultFormatType->DefaultTextFormatType,
 HyphenationOptions->{"HyphenationCharacter"->"\[Continuation]"},
 LanguageCategory->"Formula",
 ScriptLevel->0,
 SingleLetterItalics->True,

 MenuSortingValue->1700,
 FontFamily->"Times",
 FontSize->14,
 FontColor->GrayLevel[0.2],
 UnderoverscriptBoxOptions->{LimitsPositioning->True}]

IMO the default input style formatting is an oversight or error given that this cell style is almost always used for, or to try and obtain, traditional form text formatting. In the screen grab below you can see on top a cell created with the default DisplayFormula style which has an underlying Cell[BoxData[ ... structure. After editing the stylesheet you create a new cell and begin typing and you have a Cell[TextData... structure (equation below).