If you absolutely need to delete any of the videos due to slow connectivity/lack of storage space, simply delete the app off your device. This will clear the cache of any data and videos you may have downloaded via the app. Then, you can re-install the app to find that all videos have been deleted. Please note that deleting the app will NOT delete your course progress! The Themis app is continuously syncing with the online database so rest assured all your progress is still being tracked!
Although Themis does not have a button to manually delete videos off your device, there is a slider under "Account" that will allow you to automatically remove videos after watching anywhere from 40% to 100% of the video lecture.