Combination of Automattic's _s theme and Bootstrap 4 by holger1411.
http://understrap.com
Frameworks · WordPress Theme
Get access to data on 18,199 websites that are UnderStrap Customers. We know of 9,013 live websites using UnderStrap and an additional 9,186 sites that used UnderStrap historically and 4,359 websites in the United States.
Get a list of 18,199 websites using UnderStrap which includes location information, hosting data, contact details, 9,013 currently live websites and an additional 8,061 domains that redirect to sites in this list. 9,186 sites that used this technology previously and 4,359 websites in the United States currently using UnderStrap.