My themes page doesn't have an "add new" button

If you do not have an "Add New" button on your Themes page, you are likely using

Unfortunately, our themes are not compatible with, because does not permit third-party themes to be uploaded on their platform (among many other limitations). Even with a custom domain or premium account, themes cannot be uploaded onto a blog.

If you'd like to switch to a self-hosted WordPress site, here is the moving guide from WordPress.