Mean-field Gaussian chain theory for nondilute polymer solutions is being applied to polymer chains in the theta condition confined to a slit in a wide range of concentrations. Various existing lattice chain theories were used to obtain the effective potential for the Gaussian chain. Calculation results for the monomer density profile, the partition coefficient, and the chain dimension across the slit at various concentrations were computed and compared with the results obtained in the lattice Monte Carlo simulations. The need for a higher concentration to flatten the density profile and to cause the weak-to-strong penetration transition, compared with athermal chains, was confirmed. At the monomer-monomer interaction widely used as the theta condition on the cubic lattice, the chain cluster theory by Freed and the random mixing approximation by Huggins provide a better agreement with the simulation results than the Flory model and the Guggenheim model do. The agreement is, however, not as good as the one for athermal chains.