Moment of inertia of a thin square plate about an axis passing through its diagonal is I. Its moment of inertia about an axis passing through its centre in the plane of the plate and making an angle $\mathrm{\theta }$ with the diagonal is

Given I is the MI about a diagonal of the square. 

According to perpendicular axis theorem, 

MI about an axis normal to the plane will be 2I (sum of the MIs about two diagonals.)

Similarly, if I' is the MI about the axis making an angle $\mathrm{\theta }$, MI about the normal axis will be 2I'

Here 2I' = 2I, so the moment of inertia of the said axis is also I.

M.I is independent of angle $\mathrm{\theta }$.