H1 Font size issue on mobile. Change size with custom css.

Hi, I’m trying to reduce the size of the font on mobile. The theme is responsive but then a big bold h1 title appears and takes all the space on mobile. Also, the words are cut in 2.

I tried to apply these set of css on the child theme style.css but without success :

@media only screen and (max-width: 767px)
h1 {
font-size: 20px;
line-height: 1;
}

Do you have any idea? Also where should I place my custom css ? In the style.css of the child theme or the child-theme.css one ? Thanks!!

And! The theme is really awesome ! Thanks for such a great flexibility!
Benjibenja