If you are using a third party hosting site like iPage, Bluehost, Netfirm, HostGator and etc, they restrict the ability to import custom theme to PRO account users. If your current hosting plan do not allow you to use custom theme, you may need to contact your hosting service to see what features is available to you or if you need to upgrade before you can upload a custom theme.
don’t have any restriction on custom theme on any plan level including free accounts. So Weebly.com
is an option if you want to be able to upload a custom theme, but don’t want to upgrade your current hostingplan.