The equations of gravitational modes of stratified, incompressible and ideally conducting fluids, with a viscous tensor in a sheared magnetic field are given. Boundary conditions at fluid discontinuities are derived. Properties of the frequency spectrum are obtained from variational principles. A detailed description of the modes and the Rayleigh-Taylor instability is given for a plasma-vacuum interface, with different magnetic fields on each side, in three cases: isotropic viscous properties, vc>> omega ci( omega ci, ion cyclotron frequency; vc, ion collision frequency) and non isotropic viscous properties in the limit omega ci>>vc for k//B0, which is dominated by dissipative forces and for k perpendicular to B0, which is governed by gyroscopic forces. Applications of the theory to accelerated plasmas are discussed.