Option Explicit

Sub Macro1()
Dim prevSent As Range
Dim sentNumb As Long
    If do_DocVars_Exist = True Then
        sentNumb = ActiveDocument.Variables("selNumb").Value
        If ActiveDocument.Sentences.Count >= sentNumb Then
            Set prevSent = ActiveDocument.Sentences(sentNumb)
            With prevSent
                .Select
                .Font.Size = ActiveDocument.Variables("selSize").Value
                .Font.Color = ActiveDocument.Variables("selColor").Value
                .Font.Name = ActiveDocument.Variables("selName").Value
            End With
        Else
            MsgBox "There is no sentence number " & sentNumb
        End If
    Else
        MsgBox "The required variables are not present"
    End If
End Sub

Function do_DocVars_Exist() As Boolean
Dim oVar As Variable
Dim iVar As Integer: iVar = 0

    For Each oVar In ActiveDocument.Variables
        Select Case oVar.Name
            Case "selNumb", "selSize", "selColor"
                If IsNumeric(oVar.Value) Then
                    iVar = iVar + 1
                End If
            Case "selName"
                iVar = iVar + 1
        End Select
    Next oVar
    If iVar = 4 Then do_DocVars_Exist = True
End Function