Generally, what’s inside your unit is treated as your property, so you are free to do with it as you please eg by changing the curtains into shutters.
(Are you in NSW? This page outlines what is common property ie not your property:
And the answer to the question about insurance on this page [“Do I have to insure the contents of my strata unit?]” specifically includes curtains in the list of items that are not common property:
However, if you are attaching the shutters to the wall or the windows, an argument could be made that by so attaching, they become part of the common property, and therefore the responsibility of the Owners Corporation (which DOESN’T want that responsibility!)
Also, does your strata scheme have any rules / bylaws about external appearances of the block, and will the shutters be very plainly not like everyone else’ window coverings?
Best to ask your strata executive or strata manager, as you say, so as not to get caught out.