Hi Artur,

for example using custom CSS

@media screen and (max-width: 680px) {
.article-details .article-header h1, 
.article-header h1 {
  font-size: 1.5em;}
}

remember that you have compressed CSS right now, so disable CSS compression, add new lines - compress again - and clear browser and joomla cache to see difference.

CSS is 100% OKAY!