  Mesoscopic density functional theory for inhomogeneous mixtures of sperical
particles is developed in terms of mesoscopic volume fractions by a systematic
coarse-graining procedure starting form microscopic theory. Approximate
expressions for the correlation functions and for the grand potential are
obtained for weak ordering on mesoscopic length scales. Stability analysis of
the disordered phase is performed in mean-field approximation (MF) and beyond.
MF shows existence of either a spinodal or a $\lambda$-surface on the
volume-fractions - temperature phase diagram. Separation into homogeneous
phases or formation of inhomogeneous distribution of particles occurs on the
low-temperature side of the former or the latter surface respectively,
depending on both the interaction potentials and the size ratios between
particles of different species. Beyond MF the spinodal surface is shifted, and
the instability at the $\lambda$-surface is suppressed by fluctuations. We
interpret the $\lambda$-surface as a borderline between homogeneous and
inhomogeneous (containing clusters or other aggregates) structure of the
disordered phase. For two-component systems explicit expressions for the MF
spinodal and $\lambda$-surfaces are derived. Examples of interaction potentials
of simple form are analyzed in some detail, in order to identify conditions
leading to inhomogeneous structures.
