Hi
I don't now why two column in mobile view

Cześć Joachim.

Thank you for contacting us.
Try this code:

@media only screen and (min-width:320px) {
.joms-module .joms-list--thumbnail .joms-list__item {
	width: 100%;
}
}
@media only screen and (min-width: 768px) {
.joms-module .joms-list--thumbnail .joms-list__item {
	width: 25%;
}
}

Cześć Michał!
I added this code to the file override.css, but does not work

Cześć Joachim.

You use plg_jchoptimize that merges and caches css files.
Please, disable it, clear Joomla! cache - then check if that file is loaded.

hm, I turned off the plg_jchoptimize, cleaned the cache, but it does not change :(
I do not know what I'm doing wrong

Cześć Joachim.

No, you did just fine: snag.gy/R4oeCg.jpg
On mobile it should have one column.

Hi
Problem solved: from override.css it was necessary to remove the code:

.joms-sidebar {
  width: 37.197%;
}
 
.joms-main {
  width: 62.803%;
}