  We propose a field-theoretical approach based on the thermodynamic
perturbation theory and within it derive a grand thermodynamic potential of the
inhomogeneous ionic fluid as a functional of electrostatic potential for an
arbitrary reference fluid system. We obtain a modified Poisson-Boltzmann
equation as the Euler-Lagrange equation for the obtained functional. Applying
Noether's theorem to this functional, we derive a general mean-field expression
for the stress tensor consistent with the respective modified Poisson-Boltzmann
equation. We derive a general expression for the macroscopic force acting on
the dielectric or conductive body immersed in an ionic fluid. In particular, we
derive a general mean-field expression for the disjoining pressure of an ionic
fluid in a slit pore. We apply the developed formalism to describe three ionic
fluid models of practical importance: nonpolarizable models (including the
well-known Poisson-Boltzmann and Poisson-Fermi equations), polarizable models
(ions carry nonzero permanent dipole or static polarizability), and models of
ion-dipole mixtures (including the well-known Poisson-Boltzmann-Langevin
equation). For these models, we obtain modified Poisson-Boltzmann equations and
respective stress tensors, which could be valuable for different applications,
where it is necessary to estimate the macroscopic forces acting on the
dielectric or conductive bodies (electrodes, colloids, membranes, {\it etc}.)
together with the local electrostatic potential (field) and ionic
concentrations.
