If a header isn’t specified, the header is using the same font as the body. Note you don’t have to specify the same font for the headers in the customizer, they will inherit the body font automatically.
Altitude:
Body: PT Sans
Header: Philosopher
Amazon:
Body: Trebuchet MS
Header: Georgia
Beach:
Body: Arial
Header: Georgia
BigBang:
Body: Exo
BlueSky:
Body: Open Sans
City:
Body: Helvetica
Header: Arial Narrow
CitySlicker:
Body: Open Sans
EyeOne:
Body: Raleway
Harp:
Body: Lato
Horizon:
Body: Lucida Sans
Island:
Body: Open Sans
QuiltWork:
Body: Open Sans
Sidewall:
Body: Lato
Sky:
Body: Trebuchet MS
Street:
Body: Helvetica
Syndicate:
Body: Roboto Condensed
Syndicate 2:
Body: Roboto Slab
Urban:
Body: Questrial
Wave:
Body: Trebuchet MS
Window:
Body: Open Sans