Howspace uses Bootstrap 4 library and that has the following font definition:
-apple-system,BlinkMacSystemFont,"Segoe UI",Roboto,"Helvetica Neue",Arial,"Noto Sans",sans-serif,"Apple Color Emoji","Segoe UI Emoji","Segoe UI Symbol","Noto Color Emoji"
So this will try to use "-apple-system" font, but if that is not found from your computer then "BlinkMacSystemFont", then "Segoe UI" and so on.
If none of those fonts is found from your computer then the default "sans-serif" font is used.
This can be a bit confusing, but the advantage is that Howspace doesn't have to load any webfonts so this design makes the pages loads faster.