Layer background has background image
Background image with origin point in the middle
Full screen scale outer
every tick
background image set position to
X: (ViewportBottom("background") - ViewportTop("background")) / 2
Y: (ViewportRight("background") - ViewportLeft("background")) / 2
background image set size
Width: ViewportRight("background") - ViewportLeft("background")
Height: ViewportBottom("background") - ViewportTop("background")
This should dymanically resize the background to fill your browser.