Oh, you want to change the *default* fill color for text boxes. Then
you have to open your template as a document (via File>Open), create a
text box, specify the desired settings and make these the default.
Note that there is no way to specify default settings just for text
boxes. Instead, your changes applies to all autoshapes (including
callouts and arrows, for example). Anyway, here is how to do it: With
the cursor in the text box, click Draw on the Drawing toolbar. Click
Set AutoShape Defaults.
After you have changed the defaults, you can delete the text box in
the template (unless of course you actually want one there). Finally,
save and close the file.