The full text of most Rice dissertations and theses is available online in the Digital Scholarship Archive at scholarship.rice.edu.
Print copies are available in the library basement.
To locate theses and dissertations in the library catalog you can:
Search by author or by title keyword. If you are looking for dissertations from a particular department, do a keyword search: type rice thesis and department name. Example: rice thesis and chemistry.
Searching by subject will seem to work but it is not the best choice because current dissertations do not have Subject Headings assigned.