Installing a new theme for Magento 2 store is necessary and should be one of the tasks in priority. That is easy to find many beautiful themes in Magento Marketplaces or through searching tools. You can assign installation task to the theme vendor or install a new theme by yourself by following their installation guide and user guide.
The theme will help you make your store become user-friendly, professional and attractive to vistors. Choosing a suitable theme also enhances the appearance of your store and build your own brand name. Therefore, following some simple steps which are given bellow to install a new theme for your Magento store right now.
OPtion 2: Find the Magento 2 theme under Magento Marketplace
Find Partners & Extensions.
Visit Magento Marketplaces.
Stores > Settings > Configuration.
On the Admin sidebar,
Content > Design > Configuration > Select your current theme > .
Design Themefield, select the new Theme as you want
Ref: Magento 2 User Guide
It comes to the end of tutorial: How to Install a New Theme in Magento 2.