I recommend you Themeforest:
The thing I like about Themeforest is that it specifies the specific Magento versions that are compatible for each of the themes.
Thanks, @Gabriel Guarino
The only problem is that all these themes are paid. I need something to test first store on CE2.1.
You can find suitable Magento 2 Theme from this article : Top Free & Multipurpose Magento 2 Theme
You can use the Luma theme, which is included by default in Magento.