This paper presents a modification of General Relativity within the framework of Horndeski scalar-tensor theories. We postulate the existence of a scalar field that is universally coupled to the matter metric via a conformal factor. This approach ensures the strict equality of the propagation speeds of gravitational and electromagnetic waves (cg = c_gamma), satisfying the stringent constraints from the GW170817 event. To reconcile the theory with Solar System tests, a Cubic Galileon term is introduced in the Lagrangian, implementing the Vainshtein screening mechanism. We show that by selecting the energy scale of the theory as Lambda ~ 10^-13 eV, the scalar field is screened within planetary systems but generates an effective "fifth force" on galactic scales, mimicking a Dark Matter halo and leading to flat rotation curves. On cosmological scales, the field dynamics reproduce the accelerated expansion of the Universe (attractor w approx -1), offering a unified solution to the Dark Matter and Dark Energy problems.