changing H3 font not working for ecommerce page

Site URL: https://saffron-lavender-8drt.squarespace.com/config/commerce

Hello, 
I changed my title font in css, with -> "hirakakustd-w8" and it work perfecty for most of the pages, but on the e-commerce landing page, and on the detail of the product its not working. I don't understand why ..

Also i was wondering if it is possible ton change the thickness of the buttons. They are too bold ! 

Here is the website, its not published yet : https://saffron-lavender-8drt.squarespace.com/config/commerce
I hope you'll help me, MANY THANKS  ! 

Zoé