alceste The problem with H1 and H2 is also that in other places of the web they appears with the same size. H2 has the same size than H1 and this is not normal, I think. See the screenshot..
Upon debugging, Could see that H2 tag has font-size: 42px which is almost similar as 44px - set for H1 tag, henceforth, there isnt much notable difference in terms of appearence
Try add this to override.css file and change font-size value accordingly :
h2 {font-size:28px !important;}
Kindly ensure to clear your browser cache if changes dont reflect.