How override a default phtml in Magento 2?

I want to update/improve the theme "magento\_plushe".
I add a file on `app\design\frontend\magento_plushe\Magento_Theme\html\header.phtml`
But Magento ignore this file and load `app\code\Magento\Page\view\frontend\html\header.phtml`

Configuration: 
-Magento 2.0.0.0-dev55
-Cache disabled