  The Diffuse Supernova Neutrino Background (DSNB) -- a probe of the
core-collapse mechanism and the cosmic star-formation history -- has not been
detected, but its discovery may be imminent. A significant obstacle for DSNB
detection in Super-Kamiokande (Super-K) is detector backgrounds, especially due
to atmospheric neutrinos (more precisely, these are foregrounds), which are not
sufficiently understood. We perform the first detailed theoretical calculations
of these foregrounds in the range 16--90 MeV in detected electron energy,
taking into account several physical and detector effects, quantifying
uncertainties, and comparing our predictions to the 15.9 livetime years of
pre-gadolinium data from Super-K stages I--IV. We show that our modeling
reasonably reproduces this low-energy data as well as the usual high-energy
atmospheric-neutrino data. To accelerate progress on detecting the DSNB, we
outline key actions to be taken in future theoretical and experimental work. In
a forthcoming paper, we use our modeling to detail how low-energy
atmospheric-neutrino events register in Super-K and suggest new cuts to reduce
their impact.
