While this may be a nice exercise to learn bootc, shipping a whole OCI image, just because you wanted to put a couple files under /usr seems quite wasteful to me.
To put things into perspective, GTK themes, unless they bring lots and lots of bitmap images (which doesn't happen nowadays), rarely exceed a megabyte in size.
I guess one could spend less time learning how to package these as RPM packages and set up COPR to do just that, making OCI + bootc entirely optional (and yet you could build an OCI + bootc installing this package if you so wished!).