Looks like the site search bar is turned on in the Site Identity configuration page. For this theme, there is a search form already in the custom code, so the Site Identity search bar option should not be activated.
This is mentioned in the read-me file that comes with the theme package. I also put this file on the site here as a page for reference — Mittwoch.
See if the site searchbar option is turned on, on your Site Identity config page, and if so, if turning it off fixes the display problem for the theme.