Windows can be created and drawn two different ways. One is the default Gui. The default is drawn using the guiTex.png graphic. The appearance of the default Gui is similar to a regular Windows Gui. Most default-drawn Gui elements must be added through a Window instance rather than the GuiManager. It is common practice to create a Window through the GuiManager, then to add the children Gui elements through the newly-created Window.
To chnage the appearance of the Gui, most Gui elements can be constructed using SpriteFrames. SpriteFrame-created GuiElements can be created either through the GuiManager or a Window instance.
FlatRedBall.Gui.GuiManager.AddWindow() FlatRedBall.ManagedSpriteGroups.SpriteFrame