The digital transformation in education challenges teacher education. While the use of both subject-specific and generic digital tools for teaching is part of most teacher training in all subjects nowadays, reliable and adaptable computer science (CS) knowledge, which forms the basis for the effective and efficient use of the tools, is not. Also, few training courses cater to the needs of in-service teachers, especially considering their limited resources and lack of basic knowledge of computer science. In this paper, we present a literature review identifying the fundamental computer science skills required for teaching. We also address the challenge of limited resources for the professional development of in-service teachers and propose design principles for effective teacher training courses in this field. As a result, we developed a competency grid with competencies of seven basic CS topics together with four design principles for corresponding teacher training courses.