Sorry but you asked about whole content so we gave you css. If this does not work as you want, then you must remove this css and then write separately css for each element where you want to change font because this can`t be done other way.
CSS files can`t be touched or modified. All modifications should be done in Theme options > Layout > Custom CSS & JS > Custom CSS section. Only then you will be able to make theme updates and all modifications will be safe in database.
Comments
for that, you can use below css:
#Wrapper #Content { font-size: 12px !important; font-family: FONT_NAME !important; }