Hello Sizuka, and welcome to Seven Forums.
I have not seen a way to prevent this from being changed when you use a new theme.
As a workaround, see if you can save the theme with the font changes first. Next, change your theme manually
and not with a themepack, and save this theme again. Are the changed font's still in the new current theme this way? If so, then you can just right click on the other theme to delete it.