A fairly standard game window could typically use these settings:

[Display]
ScreenWidth     = @MainCamera.FrustumWidth
ScreenHeight    = @MainCamera.FrustumHeight
Title           = Borderless Test
FullScreen      = false
 
[MainCamera]
FrustumWidth    = 320
FrustumHeight   = 240
FrustumFar      = 2.0
FrustumNear     = 0.0
Position        = (0.0, 0.0, -1.0)

By setting the Decoration property of the Display section to false (true by default) you will remove the windows borders, titlebar, icon and buttons:

[Display]
ScreenWidth     = @MainCamera.FrustumWidth
ScreenHeight    = @MainCamera.FrustumHeight
Title           = Borderless Test
FullScreen      = false
Decoration 	= false

You can make the borderless window the same size as the current screen by omitting the ScreenWidth and ScreenHeight properties, and if Decoration is set to false:

[Display]
Title           = Borderless Test
FullScreen      = false
Decoration 	= false

Note: This is not a true fullscreen mode, but rather, the window sized to the entire screen.