You need to create registration.php to register your theme.

Registration.php file code :

<?php
\Magento\Framework\Component\ComponentRegistrar::register(
                \Magento\Framework\Component\ComponentRegistrar::THEME,
                'frontend/Vendorname/themename',
                __DIR__
);

After that clear the cache by running cache:clean command and check it will show in design configuration setting dropdown

Also from admin click on Content->Design->Configuration here you can see your custom theme

You need to create registration.php to register your theme.

Registration.php file code :

<?php
\Magento\Framework\Component\ComponentRegistrar::register(
                \Magento\Framework\Component\ComponentRegistrar::THEME,
                'frontend/Vendorname/themename',
                __DIR__
);

After that clear the cache by running cache:clean command and check it will show in design configuration setting dropdown

Also from admin click on Content->Design->Configuration here you can see your custom theme

For more details refer this link - http://devdocs.magento.com/guides/v2.1/frontend-dev-guide/themes/theme-create.html