Don't login by entering your credentials into the login overlay box, click on its arrow symbol to get to the login page. On that page you can set your session duration to infinite.
This "fancy" bootstrap-blocky-design is sometimes not that intuitive. And the most annoying fact about that theme, at least the mobile version, didn't check in a desktop browser yet, it doesn't show the latest post of threads in the overview, it shows only the thread starter. Especially annoying if you clicked that checkmark button by accident.