We develop an energy landscape based mean-field theory for thin films confined between parallel substrates. We use the approach to explore how the dimensions of the film and the interactions between the film and the substrates impact the equilibrium phase diagram and the ideal glass transition. The theoretical predictions are in qualitative agreement with the experimentally observed trends for confined fluids. We also use the theory to determine how the average pressure tensor of minimum energy configurations (i.e., inherent structures) depends on film density. This is, to our knowledge, the first theoretical prediction of an "equation of state of an energy landscape" for thin films. It suggests how the intrinsic mechanical properties of thin-film glasses derive from the physical dimensions of the sample, the interactions between the film and the substrates, and the balance between internal cohesive and packing forces.