When you open the Customize page, you’ll see there’s a new feature called Typography. Click on it and then proceed to Theme Typography. You’ll see the Main Manu tab you created, now you can edit it as you like. You’ll have pretty much the whole Google Fonts installed now.
Thank you for your answer. But the thing is that something is preventing me from changing the font size. Using Google fonts I also managed to change the font style, font weight etc (the same I did in code) but the font size still doesn’t change under the appearance tab. Is there a file in my theme preventing this change or something ?