Combination of Automattic's _s theme and Bootstrap 4 by holger1411.
http://understrap.com
Frameworks · WordPress Theme
Get access to data on 13,048 websites that are UnderStrap Customers. We know of 7,314 live websites using UnderStrap and an additional 5,734 sites that used UnderStrap historically and 3,544 websites in the United States.
Get a list of 13,048 websites using UnderStrap which includes location information, hosting data, contact details, 14,014 currently live websites which includes 6,700 domains that redirect to sites in the list. 5,734 sites that used this technology previously and 3,544 websites in the United States currently using UnderStrap.