i am working on theme in magento 2,but i am getting error in terminal(CMD),the error message is  **"unable to load theme by specified key:My/Helloworld"** when i run php bin\magento setup:static-content:deploy, my required file are 
**app\design\frontend\My\Helloworld\composer.json**

```
{
    "name": "magento/theme-frontend-My",
    "description": "N/A",
    "require": {
        "php": "~5.5.0|~5.6.0|~7.0.0",
        "magento/theme-frontend-blank": "100.0.*",
        "magento/framework": "100.0.*"
    },
    "type": "magento2-theme",
    "version": "100.0.1",
    "license": [
        "OSL-3.0",
        "AFL-3.0"
    ],
    "autoload": {
        "files": [
            "registration.php"
        ]
    }
}

```

**app\design\frontend\My\Helloworld\registration.php**

```
\Magento\Framework\Component\ComponentRegistrar::register(
    \Magento\Framework\Component\ComponentRegistrar::THEME,
    'frontend/My/Helloworld',
    __DIR__
);

```