I also made some changes to your code. I improved setting the oblique factor and added a logic test for setting the active style. Note that if you click on the Go Advanced button, you can put your code in a code window to preserve formatting. Also, since your code doesn't return any object, it can be a sub rather than a function.
Code:
Sub textsty()
Dim IsoStyle As AcadTextStyle
Dim name As String
Dim Entry As AcadTextStyle
Dim Found As Boolean
Found = False
For Each Entry In ThisDrawing.TextStyles
If Entry.name = "Isom1" Then
Found = True
Exit For
End If
Next
If Not (Found) Then
Set IsoStyle = ThisDrawing.TextStyles.add("Isom1")
IsoStyle.fontFile = "romans.shx"
IsoStyle.Height = 0
IsoStyle.Width = 1
IsoStyle.ObliqueAngle = DegToRad(-30)
Else
End If
If Not ThisDrawing.ActiveTextStyle.name = "Isom1" Then ThisDrawing.ActiveTextStyle = IsoStyle
End Sub
Public Function DegToRad(Angle As Double)
'convert degrees to radians
DegToRad = ((Angle / 180) * PI)
End Function
Public Function PI() As Double
PI = Atn(1) * 4
End Function