Activity not closing on Back press when theme is Theme.NoDisplay

I imagine it is closing but you see no evidence of this because it is 'NoDisplay'. An Activity using this theme has no visible UI (hence it is not logical for such an Activity to process UI events) and should not be kept alive. See this question for example:

how to completely get rid of an activity's GUI (avoid a black screen)

If you do want this Activity's layout to be visible and to handle events, you should use a different theme. If on the otherhand this is an invisible Activity that just does some background processing, call finish() in onCreate to close it when that processing is done.