Your dictionary seems to be a bit old. :) Θέσις is the older form of the word, still valid, but not what is taught or used in everyday speech.
You can use online dictionaries that have the modern endings and declensions, like wiktionary or wordreference. Actually declensions can be very different, for example θέσις becomes θέσεως only, while θέση becomes θέσης or θέσεως (the old form is retained as an alternative).
Thank you. The original edition of the dictionary dates from 1965. I've come across other inaccuracies (confusing πώς and πως) and like to get my information from more than one source. It's nice to have a hard-copy, though, and online dictionaries are not to be trusted any more than any other.
The original edition of the dictionary dates from 1965.
That explains it then. Demotic/Standard Modern Greek became the official version of the language in 1976 (https://en.wikipedia.org/wiki/Greek_language_question#1976:_Resolution_and_the_end_of_diglossia).