What makes a Magento theme?
A Magento theme is a collection of files that define the look, layout, and other outputs from the Magento system.
Unique aspects of a Magento theme
Magento 2 themes differ from design implementations in other content management systems and e-commerce platforms in a few key ways:
Maximum ability to customize Magento
Support for multiple concurrent themes
They offer an uninterrupted workflow
They minimize debugging time for errors
Magento's theming system provides the ability to highly customize the way Magento 2 looks to your customers: you can customize every aspect of your Magento store through layout, CSS, JavaScript, templates, and local files.
The support for multiple concurrent themes is another feature that makes Magento somewhat unique in its field, allowing you to style your store differently with Magento.