Janet, you don't need a style.
For the existing occurances of 17th you can use Find/Replace to do the whole
document.
Firstly create a 17th with the ordinal as superscript, then Ctrl+C to copy
to the clipboard.
Open Find/Replace, in the Find what: type 17th, in the Replace with: type
^c. ^c is the code for the contents of the clipboard. Replace all. All
occurances of 17th will be changed.
For future occurances of 17th (and any other ordinals), as previously
described turn on the Autocorrect for the Ordinals to be formatted as
superscript.
Hope this helps
DeanH