The path difference between the reflected ray and incident ray depends upon the thickness of the air gap between lens and the base.
As the lens is symmetric along its axis, the thickness is constant along the circumference of a ring of a given radius.
Hence, Newton’s rings are circular.
At the centre, the air gap thickness is zero.
Hence the path difference is zero.
However, as the ray reflects from a denser medium, it undergoes a phase change of $π$
Thus incident ray and reflected ray interfere destructively.
Hence, centre of interference pattern is dark.