Thats a good question... ^^ I changend the theme about 20 times in the last hour to figure that out!
The most sections are used on different places...
So far I came:

style "osso-SystemFont"
#Used by the GSM/3G Switcher (See screenshot on my last post)

style "SystemFont"
# This is used by the most elements so far I can see that

style "SmallSystemFont"
#Submenu sub text (orange, see screenshot on my last post)