HI sequoiac, thanks for your answer, but I don't manage to do so...
In fact, that's the first thing I tried, I went to style settings and I changed all the fonts to the same one but my website was still loading some other fonts.
Then I created new blocks and pages that I wasn't using in my original website (for exemple a gallery section) so then I could go to styles and change the fonts from these blocks that by default were using different fonts (and consequently my website was loading them even if they didn't appear directly in any page).
The thing is that probably I'm missing some blocks that still have the default fonts selected so when my webpage is loaded it loads all the fonts that "could" be used in my website (the one I actually use + all the default fonts that I didn't manage to change).
So I don't know how I could change ALL my fonts so my webstie just have to load the ACTUAL fonts used.