Hello, @tk,
That is the main reason, you set that layout using the Custom CSS on the desktop, and that Custom CSS also affects the site layout on tablet versions as well.
And I am sorry but the way you are checking the site is not the standard way, the standard way is to check the site on real devices or on live online simulators, like: http://www.responsinator.com/?url=http%3A%2F%2Fserver166003.nazwa.pl%2F on this test you can see that your site is having the issue on 768px.
Now I have added the Custom CSS code in the media query on your home page: https://postimg.cc/G4vk2L1h and tested the site again on responsinator and check the result now: http://www.responsinator.com/?url=server166003.nazwa.pl%2F
Regards 8Themes Team.