Youi cannot format a content control dropdown list in this manner.
You could use a selection to format text in the document, or perhaps simpler use it to insert an appropriate formatted autotext entry in a bookmarked location e.g.
Code:
Private Sub Document_ContentControlOnExit(ByVal ContentControl As ContentControl, Cancel As Boolean)
Dim orng As Range
Select Case ContentControl.Range.Text
Case "High"
AutoTextToBM "bmRating", ActiveDocument.AttachedTemplate, "High"
Case "Significant"
AutoTextToBM "bmRating", ActiveDocument.AttachedTemplate, "Significant"
Case "Low"
AutoTextToBM "bmRating", ActiveDocument.AttachedTemplate, "Low"
Case Else
Set orng = ActiveDocument.Bookmarks("bmRating").Range
orng.Text = ""
ActiveDocument.Bookmarks.Add ("bmRating"), orng
End Select
lbl_Exit:
Set orng = Nothing
Exit Sub
End Sub
Private Sub AutoTextToBM(strbmName As String, oTemplate As Template, strAutotext As String)
'strBMName is the name of the bookmark to fill
'oTemplate is the template with the autotext - probably ActiveDocument.AttachedTemplate
'strAutotext is the name of the autotext entry
Dim orng As Range
On Error GoTo lbl_Exit
With ActiveDocument
Set orng = .Bookmarks(strbmName).Range
Set orng = oTemplate.AutoTextEntries(strAutotext).Insert _
(Where:=orng, RichText:=True)
.Bookmarks.Add Name:=strbmName, Range:=orng
End With
lbl_Exit:
Exit Sub
End Sub