I know, and I tried that way, the project crashed. The error message was:
java.lang.IllegalStateException: Cannot set style once stage has been set visible
This is because for the first form, that of the B4XMainPage, you have to write it in the Main form:
Sub AppStart (Form1 As Form, Args() As String)
MainForm = Form1
MainForm.SetFormStyle("TRANSPARENT")
MainForm.Show
If instead you write the following code in the B4XMainPage, the project crashes:
Private Sub B4XPage_Created (Root1 As B4XView)
B4XPages.GetNativeParent(Me).SetFormStyle("TRANSPARENT")
Root = Root1