It is possible to search for dissertations within Web of Science's platform. You will need to either select "ProQuest Digital Dissertations & Theses Citation Index"(PQDDTCI) at the top dropdown menu OR Select All databases from the same menu, and then limit to ProQuest Digital Dissertations...under the Collections dropdown menu. The Collections menu will also allow you to search combinations of the available databases, such as Web of Science, Medline AND PQDDTCI at the same time.