AS far as I know you could add as many graphdata elements to a style template as you want though most of the SAS supplied ones define or inherit 12 or fewer.
I think that you might have a bit more luck listing out a base style and examining the syntax for likely changes than wading from whole cloth through Proc Template documentation.
proc template;
source styles.normal;
source styles.meadow;
run;
will send the proc template source for the two styles listed to the log.
I included Meadow as it inherits from Normal and you can see how to override or use inheritance.
Then you would have specific syntax to search for instead of wandering through the documentation at random.