Analytic integral equation theory for treating athermal mixtures of hard spheres and polymers is extended to study thermal interaction and solvent duality effects in suspensions of infinitely dilute spherical particles in dilute and semidilute polymer solutions. Our approach applies to all sphere/ polymer size ratios, and polymer-polymer segmental attractive interactions are included using both liquid state and Flory-Huggins theory. The effect of such thermal interactions on the polymer-induced depletion potential between hard spheres is studied with emphasis on the sphere-sphere second virial coefficient and the free energy of particle insertion. Strong enhancement of the purely entropic attractive depletion interaction is found in the proximity of the polymer-solvent critical demixing point. The magnitude of this effect and the distance from the Theta temperature where it becomes important are sensitive to system specific factors.