Because of the way WordPress.com’s technical infrastructure is designed, we are not able to support uploading of custom WordPress themes on our service.
However, there are several options if you would like to change the theme of your blog:
Several of the over 180 themes on WordPress.com have customization options to change the colors, backgrounds, layouts, and header images. Learn how to activate a new theme here.
You can purchase a Custom Design upgrade for your blog, which allows you to edit the look and feel of any WordPress.com theme.
You can move your WordPress.com blog to a web host that allows editing and uploading of completely custom themes. You can find suitable hosts here.