A unified non-Markovian theory for the vibrorotational Raman and infrared spectra of polar diatomic molecules diluted in nonpolar fluids is presented. From this theory, the physical basis of the spectra can be interpreted in terms of a few molecular properties of the isolated diatomic and of the time autocorrelation functions determining the collective effects of the solvent molecules on the vibrorotational dynamic of the diatomic. The spectrum is obtained as a diagonal part, constituted by an additive superposition of lines accounting for the integrated intensity, and an (exact) nondiagonal part accounting for the redistribution of intensity due to interbranch and intrabranch mixing effects. This theory generalizes previous theoretical frames based on a secular contribution modified by an (approximate) interference term. Also it allows the comparative analysis of the Raman and infrared spectra, and gives a clear and consistent interpretation of the theoretical lines building up the spectra.