I am running KUbuntu 20.04 and changed my default fonts there. The KDE system settings GUI has a button to configure GTK appearance, but this only allows changing the whole theme to another predefined theme.
I found two CLI utilities, gtk-chtheme and gtk-theme-switch2. One of them (I don't get which one) helped me apply the change to GTK2 apps.
But I am still stuck when it comes to GTK3 apps (the majority !).
I understand GTK switched from so-called RC files to their own flavour of CSS, but most information I can find seems directed at people creating whole new themes, which is way too complicated and overkill for me.
It does not help that search results (including here) are plagued with ultra old questions.
I am also intrigued by where Thunderbird gets it default fonts (Tinos, Arimo, Cousine) from, but that may be another question (or maybe not !).