Link to live document with the science themes and brief descriptions and links to the top 2-page killer apps.
Tracing the signatures of life and the ingredients of habitable worlds: Trace the trail of water from interstellar clouds, to protoplanetary disks, to Earth itself in order to understand the abundance and availability of water for habitable planets.
(Charting the) Rise of Metals, Dust, and the First Galaxies:
Trace the dust and metal enrichment history of the Universe. Probe the first cosmic sources of dust, the properties of the earliest star formation, and the birth of galaxies.
Revealing the interplay between stars, black holes, and interstellar matter over cosmic time*: Understand the connection between BH growth and SF, obtain a comprehensive view of the multiphase ISM, and measure the feedback from SF and AGN over cosmic time.
*(suggested alternative 3rd theme: Understanding ((or Unraveling or Unveiling)) the Co-Evolution of Black Holes and Galaxies over Cosmic Time)
Development of these themes from the top "killer apps" and the association of each killer app with a theme can be found in this (editable) Google Doc: Science Themes. Here is a PDF of the Science Themes and grouping with killer app proposals.