Many people prefer psychosocial interventions for mental health care or other concerns, but these interventions are often complex and unavailable in settings where people seek care. Intervention designers use technology to improve user experience or reach of interventions, and HCI researchers have made many contributions toward this goal. Both HCI and mental health researchers must navigate tensions between innovating on and adhering to the theories of change that guide intervention design. In this paper, we propose a framework that describes design briefs and evaluation approaches for HCI contributions at the scopes of capabilities, components, intervention systems, and intervention implementations. We show how theories of change (from mental health) can be translated into design briefs (in HCI), and that these translations can bridge and coordinate efforts across fields. It is our hope that this framework can support researchers in motivating, planning, conducting, and communicating work that advances psychosocial intervention design.