ah ok I think I understood the issue already

When I downloaded your latest package, it replaced the old package which is removed from the system hence running the pre-removal script which states to revert to blanco theme...

I think you should keep it as for example someone uninstalled your package the system should revert to blanco for them...

You may just want to remove the /themes/example line from both postint and prerm as you that's for Vesuri's example theme...You may want to replace it with your themes if you wish