H. Douglas Brown defines theme-based instruction as a “weak version” of content-based instruction (288). The difference is slight, but it really boils down to priorities and course objectives. The primary purpose of CBI is to teach language learners a subject, such as math, engineering, biology, etc. in the target language. If the students do not learn the content, the course will not have met its objectives. In a theme-based instruction course, language instruction is elevated and prioritized to equal status with the content.
Brown, H. Douglas. Teaching by Principles. Pearson, 2007.