You cannot use CSSUtils for this. You need to use a CSS file for this:
B4X:
Sub AppStart (Form1 As Form, Args() As String)
MainForm = Form1
MainForm.SetFormStyle("UNIFIED")
MainForm.RootPane.LoadLayout("1")
MainForm.Show
ListView1.Id = "lv"
MainForm.Stylesheets.Add(File.GetUri(File.DirAssets, "lv.css"))
For i = 1 To 100
ListView1.Items.Add(i)
Next
End Sub