That's simple: Add a theme to the board. Choose it with another browser and nickname, now with the browser that you use to go to the acp, go there and delete the theme. Now try to connect to the site with the test nickname (you could do it with your main browser and admin nickname but it will be more difficult to go in the site). The error is something like "Could not get data for theme id []".
Ah now I got your point... so the problem is related to the style stored in the cookie which is validated only when chosen... but of course if the theme is then deleted the cookies is not valid anymore.
I should have fixed it adding a new check when reading the cookie.