Hi Thibaud,
These headings are declared with a font family called "Unica One", cursive; - not the default font family as other text.
You can open the file: /templates/ja_brickstore/less/variables.less
and look for this variable:
@font-family-cursive: "Unica One", cursive;
@headings-font-family: @font-family-cursive;
and change to your desired font family.
Then backup your site + compile less to css: https://www.t3-framework.org/documentation/bs3-configuration#compile-less