How to modify the fontSize value in the theme and then reapply, if not, how to explain the [qlik.app.theme.save(id)] function?

[xunziheng]

I want to modify the fontSize value of charts, which can be achieved by installing a new theme, but I see that there is a save api here(qlik.app.theme.save(id) ), I want to know what this is for, can it help me?

Thanks for the guidance!