In the 80's Kudla and Millson introduced a theta function in two variables, nowadays known as the Kudla--Millson theta function. This behaves as a Siegel modular form with respect to one variable, and as a closed differential form on an orthogonal Shimura variety with respect to the other variable. In this talk we show that the Lefschetz decomposition of (the cohomology class of) this theta function provides simultaneously the modular decomposition in Eisenstein, Klingen and cuspidal parts. Time permitting, we will report on geometric applications. This is joint work with J. Bruinier.