On my phone the page is responsive in that the buttons reposition themselves immediately when I rotate the phone.
What problems are you seeing?
As far a using the whole device size, I was surprised to find that putting
app.SetScreenMode("Game");
in the OnStart function had no effect.
I am not sure whether that is a general limitation of HTML apps and I have no other suggestion for this.
I assume the responsive CSS is overriding the attempt to resize the device.